Aerodynamics

@article{doi101090qam42889,
    author = "Cole, Julian D.",
    title = "On a quasi-linear parabolic equation occurring in aerodynamics",
    year = "1951",
    journal = "Quarterly of Applied Mathematics",
    url = "https://doi.org/10.1090/qam/42889",
    doi = "10.1090/qam/42889",
    openalex = "W2565432921",
    references = "doi101007bf03046993, doi101016s0065215608701005, doi10117515200493191543163srrotm20co2, openalexw2603003341"
}

Summary Parameters used to characterize the course of growth are described, and calculated growth parameters are presented for 105 species of birds of many taxonomic groups from a wide range of geographical localities. Growth parameters are found to exhibit as much as 20% variation within a species with respect to geographic locality and time of the nesting season. There is also considerable local variation, irrespective of season and locality, which is related to nutrition and perhaps to an inherited variability. The application of curve‐fitting as a method of analysing intraspecific variation is discussed briefly, and the importance of comparative growth studies is emphasized. Growth patterns are correlated with other parameters of the life‐history to evaluate the extent of diversity in the course of growth. Low rates of growth and prolonged growth periods occur primarily in species large for their families and in oceanic species. In most others, high rates of growth are maintained for longer periods of time. The shape of the growth curve is not related to the mode of development (i.e. whether precocial or altricial). Overall relative, or weight‐specific growth rates, as measured by the constants of fitted growth equations, are most highly correlated with the adult body size of the species, changing as the ‐0–278 power of adult body weight. Smaller variations in the rate of growth appear to be correlated with differences in nesting success; open‐nesting passerines grow faster than hole‐nesting species of a similar size. Growth rate is further correlated with brood size. Oceanic species with single egg clutches and tropical land‐birds with small clutches have low growth rates. The asymptote of the growth curve of the young (in relation to the adult weight) is related to the foraging behaviour of the adults. Aerial feeders generally have high asymptotes while those of ground feeding species are usually below adult weight. These differences are related to the need in the former for well‐developed flight at the time of fledging. The diversity of growth patterns is related to evolutionary trends which are the result of (1) selective forces acting at stages of the life‐history cycle other than development, (2) factors which affect the survival of offspring during the growth period, and (3) adjustments made to balance the energy budget of the family group. The last trend is discussed in detail in relation to the correlations found in the analysis. Two hypotheses are presented. Firstly, in species which cannot gather enough food to support even one young at a normal growth rate, the pace of development is reduced to decrease the energetic requirements of the young. Secondly, in species with small clutches, where adjustments to feeding capacities are not readily made by changing brood size, growth rate may be adjusted to accomplish this. The lack of critical energetic data to test these hypotheses is emphasized as a major deficiency in our understanding of the breeding biology of birds.

@article{doi101111j1474919x1968tb00058x,
    author = "Ricklefs, Robert E.",
    title = "PATTERNS OF GROWTH IN BIRDS",
    year = "1968",
    journal = "Ibis",
    abstract = "Summary Parameters used to characterize the course of growth are described, and calculated growth parameters are presented for 105 species of birds of many taxonomic groups from a wide range of geographical localities. Growth parameters are found to exhibit as much as 20\% variation within a species with respect to geographic locality and time of the nesting season. There is also considerable local variation, irrespective of season and locality, which is related to nutrition and perhaps to an inherited variability. The application of curve‐fitting as a method of analysing intraspecific variation is discussed briefly, and the importance of comparative growth studies is emphasized. Growth patterns are correlated with other parameters of the life‐history to evaluate the extent of diversity in the course of growth. Low rates of growth and prolonged growth periods occur primarily in species large for their families and in oceanic species. In most others, high rates of growth are maintained for longer periods of time. The shape of the growth curve is not related to the mode of development (i.e. whether precocial or altricial). Overall relative, or weight‐specific growth rates, as measured by the constants of fitted growth equations, are most highly correlated with the adult body size of the species, changing as the ‐0–278 power of adult body weight. Smaller variations in the rate of growth appear to be correlated with differences in nesting success; open‐nesting passerines grow faster than hole‐nesting species of a similar size. Growth rate is further correlated with brood size. Oceanic species with single egg clutches and tropical land‐birds with small clutches have low growth rates. The asymptote of the growth curve of the young (in relation to the adult weight) is related to the foraging behaviour of the adults. Aerial feeders generally have high asymptotes while those of ground feeding species are usually below adult weight. These differences are related to the need in the former for well‐developed flight at the time of fledging. The diversity of growth patterns is related to evolutionary trends which are the result of (1) selective forces acting at stages of the life‐history cycle other than development, (2) factors which affect the survival of offspring during the growth period, and (3) adjustments made to balance the energy budget of the family group. The last trend is discussed in detail in relation to the correlations found in the analysis. Two hypotheses are presented. Firstly, in species which cannot gather enough food to support even one young at a normal growth rate, the pace of development is reduced to decrease the energetic requirements of the young. Secondly, in species with small clutches, where adjustments to feeding capacities are not readily made by changing brood size, growth rate may be adjusted to accomplish this. The lack of critical energetic data to test these hypotheses is emphasized as a major deficiency in our understanding of the breeding biology of birds.",
    url = "https://doi.org/10.1111/j.1474-919x.1968.tb00058.x",
    doi = "10.1111/j.1474-919x.1968.tb00058.x",
    openalex = "W1983501334",
    references = "doi101016b9781483231433500149, doi101093aibsbulletin5112b, doi101111j1474919x1947tb04155x, doi101111j155856461948tb02734x, doi1023071366368, doi1023071934545, doi1023072453, doi1023074081922, doi103382ps0250096, openalexw119319751"
}

A live laggar falcon (Falco jugger) glided in a wind tunnel at speeds between 6·6 and 15·9 m./sec. The bird had a maximum lift to drag ratio (L/D) of 10 at a speed of 12·5 m./sec. As the falcon increased its air speed at a given glide angle, it reduced its wing span, wing area and lift coefficient. A model aircraft with about the same wingspan as the falcon had a maximum L/D value of 10. Published measurements of the aerodynamic characteristics of gliding birds are summarized by presenting them in a diagram showing air speed, sinking speed and L/D values. Data for a high-performance sailplane are included. The soaring birds had maximum L/D values near 10, or about one quarter that of the sailplane. The birds glided more slowly than the sailplane and had about the same sinking speed. The ‘equivalent parasite area’ method used by aircraft designers to estimate parasite drag was modified for use with gliding birds, and empirical data are presented to provide a means of predicting the gliding performance of a bird in the absence of wind-tunnel tests. The birds in this study had conventional values for parasite drag. Technical errors seem responsible for published claims of unusually low parasite drag values in a vulture. The falcon adjusted its wing span in flight to achieve nearly the maximum possible L/D value over its range of gliding speeds. The maximum terminal speed of the falcon in a vertical dive is estimated to be 100 m./sec.

@article{tucker1970aerodynamics,
    author = "Tucker, Vance A. and Parrott, G. Christian",
    title = "Aerodynamics of Gliding Flight in A Falcon and Other Birds",
    year = "1970",
    journal = "Journal of Experimental Biology",
    abstract = "A live laggar falcon (Falco jugger) glided in a wind tunnel at speeds between 6·6 and 15·9 m./sec. The bird had a maximum lift to drag ratio (L/D) of 10 at a speed of 12·5 m./sec. As the falcon increased its air speed at a given glide angle, it reduced its wing span, wing area and lift coefficient. A model aircraft with about the same wingspan as the falcon had a maximum L/D value of 10. Published measurements of the aerodynamic characteristics of gliding birds are summarized by presenting them in a diagram showing air speed, sinking speed and L/D values. Data for a high-performance sailplane are included. The soaring birds had maximum L/D values near 10, or about one quarter that of the sailplane. The birds glided more slowly than the sailplane and had about the same sinking speed. The ‘equivalent parasite area’ method used by aircraft designers to estimate parasite drag was modified for use with gliding birds, and empirical data are presented to provide a means of predicting the gliding performance of a bird in the absence of wind-tunnel tests. The birds in this study had conventional values for parasite drag. Technical errors seem responsible for published claims of unusually low parasite drag values in a vulture. The falcon adjusted its wing span in flight to achieve nearly the maximum possible L/D value over its range of gliding speeds. The maximum terminal speed of the falcon in a vertical dive is estimated to be 100 m./sec.",
    url = "https://doi.org/10.1242/jeb.52.2.345",
    doi = "10.1242/jeb.52.2.345",
    number = "2",
    openalex = "W1921571842",
    pages = "345-367",
    volume = "52",
    references = "doi101242jeb372330, doi101242jeb493509, openalexw1511362807, openalexw1518537820, openalexw1536379056, openalexw1549441713, openalexw2135641983"
}

In a centrally condensed solar nebula, the pressure gradient in the gas causes the nebula to rotate more slowly than the free orbital velocity. Drag forces cause the orbits of solid bodies to decay. Their motions have been investigated analytically and numerically for all applicable drag laws. The maximum radial velocity developed is independent of the drag law, and insensitive to the nebular mass. Results are presented for a variety of model nebulae. Radial velocities depend strongly on particle size, reaching values on the order of 104 cm/s for metre-sized objects. Possible consequences include: mixing of solid matter within the solar nebula on short timescales, collisions leading to rapid accumulation of planetesimals, fractionation of bodies by size or density, and production of regions of anomalous composition in the solar nebula.

@article{doi101093mnras180257,
    author = "Weidenschilling, S. J.",
    title = "Aerodynamics of solid bodies in the solar nebula",
    year = "1977",
    journal = "Monthly Notices of the Royal Astronomical Society",
    abstract = "In a centrally condensed solar nebula, the pressure gradient in the gas causes the nebula to rotate more slowly than the free orbital velocity. Drag forces cause the orbits of solid bodies to decay. Their motions have been investigated analytically and numerically for all applicable drag laws. The maximum radial velocity developed is independent of the drag law, and insensitive to the nebular mass. Results are presented for a variety of model nebulae. Radial velocities depend strongly on particle size, reaching values on the order of 104 cm/s for metre-sized objects. Possible consequences include: mixing of solid matter within the solar nebula on short timescales, collisions leading to rapid accumulation of planetesimals, fractionation of bodies by size or density, and production of regions of anomalous composition in the solar nebula.",
    url = "https://doi.org/10.1093/mnras/180.2.57",
    doi = "10.1093/mnras/180.2.57",
    openalex = "W2068433364"
}

The distribution of vorticity in the wake of a hovering bird or insect is considered. The wake is modelled by a chain of coaxial small-cored circular vortex rings stacked one upon another; each member of the chain is generated by a single wing-stroke. Circulation is determined by the animal's weight and the time for which a single ring must provide lift; ring size is calculated from the circulation distribution on the animal's wing. The theory is equally applicable to birds and insects, although the mechanism of ring formation differs. This approach avoids the use of lift and drag coefficients and is not bound by the constraints of steady-state aerodynamics; it gives a wake configuration in agreement with experimental observations. The classical momentum jet approach has steady momentum flux in the wake, and is difficult to relate to the wing motions of a hovering bird or insect; the vortex wake can be related to the momentum jet, but adjacent vortex elements are disjoint and momentum flux is periodic. The evolution of the wake starting from rest is considered by releasing vortex rings at appropriate time intervals and allowing them to interact in their own velocity fields. The resulting configuration depends on the feathering parameter f (which depends on the animal's morphology); f increases with body size. At the lower end of the wake rings coalesce to form a single large vortex, which breaks away from the rest of the wake at intervals. Wake contraction depends on f; the minimum areal contraction of one-half (as in momentum-jet theory) occurs only in the limit f → 0, but values calculated for smaller insects of just over one-half suggest that the momentum jet may be a good approximation to the wake when f is small. Induced power in hovering is calculated as the limit of the mean rate of increase of wake kinetic energy as time progresses. It can be related to the classical momentum-jet induced power by a simple conversion factor. For an insect or hummingbird the usual momentum-jet estimate may be between 10 and 15% too low, but for a bird it may be as much as 50% too low. This suggests that few, if any, birds are able to sustain aerobic hovering, and that as small a value of f as possible would be necessary if the bird were to hover. Tip losses (energy cost of the vortex-ring wake compared with the equivalent momentum jet) are negligible for insects, but can be in the range 15–20% for birds.

@article{doi101017s0022112079000410,
    author = "Rayner, J. M. V.",
    title = "A vortex theory of animal flight. Part 1. The vortex wake of a hovering animal",
    year = "1979",
    journal = "Journal of Fluid Mechanics",
    abstract = "The distribution of vorticity in the wake of a hovering bird or insect is considered. The wake is modelled by a chain of coaxial small-cored circular vortex rings stacked one upon another; each member of the chain is generated by a single wing-stroke. Circulation is determined by the animal's weight and the time for which a single ring must provide lift; ring size is calculated from the circulation distribution on the animal's wing. The theory is equally applicable to birds and insects, although the mechanism of ring formation differs. This approach avoids the use of lift and drag coefficients and is not bound by the constraints of steady-state aerodynamics; it gives a wake configuration in agreement with experimental observations. The classical momentum jet approach has steady momentum flux in the wake, and is difficult to relate to the wing motions of a hovering bird or insect; the vortex wake can be related to the momentum jet, but adjacent vortex elements are disjoint and momentum flux is periodic. The evolution of the wake starting from rest is considered by releasing vortex rings at appropriate time intervals and allowing them to interact in their own velocity fields. The resulting configuration depends on the feathering parameter f (which depends on the animal's morphology); f increases with body size. At the lower end of the wake rings coalesce to form a single large vortex, which breaks away from the rest of the wake at intervals. Wake contraction depends on f; the minimum areal contraction of one-half (as in momentum-jet theory) occurs only in the limit f → 0, but values calculated for smaller insects of just over one-half suggest that the momentum jet may be a good approximation to the wake when f is small. Induced power in hovering is calculated as the limit of the mean rate of increase of wake kinetic energy as time progresses. It can be related to the classical momentum-jet induced power by a simple conversion factor. For an insect or hummingbird the usual momentum-jet estimate may be between 10 and 15\% too low, but for a bird it may be as much as 50\% too low. This suggests that few, if any, birds are able to sustain aerobic hovering, and that as small a value of f as possible would be necessary if the bird were to hover. Tip losses (energy cost of the vortex-ring wake compared with the equivalent momentum jet) are negligible for insects, but can be in the range 15–20\% for birds.",
    url = "https://doi.org/10.1017/s0022112079000410",
    doi = "10.1017/s0022112079000410",
    openalex = "W2152759371"
}

Theoretical considerations and available experimental studies are combined for a discussion on the aerodynamic mechanisms of lift generation in hovering animal flight. A comparison of steady-state thin-aerofoil theory with measured lift coefficients reveals that leading edge separation bubbles are likely to be a prominent feature in insect flight. Insect wings show a gradual stall that is characteristic for thin profiles at Reynolds numbers (Re) less than about 105. In this type of stall, flow separates at the sharp leading edge and then re-attaches downstream to the upper wing surface, producing a region of limited separation enclosing a recirculating flow. The resulting leading edge bubble enhances the camber and thickness of the thin profile, improving lift at low Re. Some of the results for bird wing profiles indicate that the complications of leading edge bubbles might even be found in the fast forward flight of birds.

@article{ellington1984the,
    author = "Ellington, Charles Porter",
    title = "The aerodynamics of hovering insect flight. IV. Aerodynamic mechanisms",
    year = "1984",
    journal = "Philosophical Transactions of the Royal Society of London. B, Biological Sciences",
    abstract = "Theoretical considerations and available experimental studies are combined for a discussion on the aerodynamic mechanisms of lift generation in hovering animal flight. A comparison of steady-state thin-aerofoil theory with measured lift coefficients reveals that leading edge separation bubbles are likely to be a prominent feature in insect flight. Insect wings show a gradual stall that is characteristic for thin profiles at Reynolds numbers (Re) less than about 105. In this type of stall, flow separates at the sharp leading edge and then re-attaches downstream to the upper wing surface, producing a region of limited separation enclosing a recirculating flow. The resulting leading edge bubble enhances the camber and thickness of the thin profile, improving lift at low Re. Some of the results for bird wing profiles indicate that the complications of leading edge bubbles might even be found in the fast forward flight of birds.",
    url = "https://doi.org/10.1098/rstb.1984.0052",
    doi = "10.1098/rstb.1984.0052",
    number = "1122",
    openalex = "W2021495108",
    pages = "79-113",
    volume = "305",
    references = "doi101002zamm19250050103, doi1010079781475713268, doi1010160376042164900041, doi101017s0022112079001774, doi101146annurevfl14010182001441, doi101242jeb591169, doi1023072530028, doi1025148674, openalexw1536379056, openalexw1565704217"
}

@misc{gauthier1984phylogenetic1,
    author = "Gauthier, J. and Padian, K",
    title = "Phylogenetic, Functional and Aerodynamic Analysis of the Origin of Birds, in Hecht, M. K., Ostrom, J. H., Viohl, G., and Wellnhofer, P., eds., The Beginnings of Birds",
    year = "1984",
    howpublished = "Eichstatt, Fruende des Jura-Museums, p. 185-198",
    note = "talkorigins\_source = {true}; raw\_reference = {Gauthier, J., and Padian, K., 1984, Phylogenetic, Functional and Aerodynamic Analysis of the Origin of Birds, in Hecht, M. K., Ostrom, J. H., Viohl, G., and Wellnhofer, P., eds., The Beginnings of Birds: Eichstatt, Fruende des Jura-Museums, p. 185-198.}"
}

@misc{kingsolver1985aerodynamics2,
    author = "Kingsolver, J. G. and Koehl, M. A. R",
    title = "Aerodynamics, thermoregulation and the evolution of insect wings",
    year = "1985",
    howpublished = "Differential scaling and evolutionary changes: Evolution, v. 39, p. 488-504",
    note = "talkorigins\_source = {true}; raw\_reference = {Kingsolver, J. G., and Koehl, M. A. R., 1985, Aerodynamics, thermoregulation and the evolution of insect wings: Differential scaling and evolutionary changes: Evolution, v. 39, p. 488-504.}"
}

@misc{norberg1985evolution3,
    author = "Norberg, U. M",
    title = "Evolution of vertebrate flight",
    year = "1985",
    howpublished = "An areodynamic model for the transition from gliding to active flight: American Naturalist, v. 126, p. 303-327",
    note = "talkorigins\_source = {true}; raw\_reference = {Norberg, U. M., 1985, Evolution of vertebrate flight: An areodynamic model for the transition from gliding to active flight: American Naturalist, v. 126, p. 303-327.}"
}

@article{doi101016030096299090674h,
    title = "Vertebrate Flight: Mechanics, Physiology, Morphology, Ecology and Evolution",
    year = "1990",
    journal = "Comparative Biochemistry and Physiology Part A Physiology",
    url = "https://doi.org/10.1016/0300-9629(90)90674-h",
    doi = "10.1016/0300-9629(90)90674-h",
    openalex = "W1554135758"
}

Abstract The aerodynamic properties of a bird’s tail, and the forces produced by it, can be predicted by using slender lifting surface theory. The results of the model show that unlike conventional wings, which generate lift proportional to their area, the lift generated by the tail is proportional to the square of its maximum continuous span. Lift is unaffected by substantial variations in tail shape provided that the tail initially expands in width along the direction of flow. Behind the point of maximum width of the tail the flow is dominated by the wake of the forward section. Any area behind this point therefore causes only drag, not lift. The centre of lift is at the centre of area of the part of the tail in front of the point of maximum width. The moment arm of the tail, about its apex, is therefore more than twice the moment arm of a conventional wing about its leading edge. The drag of the tail is a combination of induced drag proportional to lift, and profile drag proportional to surface area. Induced drag can be halved by drooping the outer tail feathers to generate leading edge suction. This may be used for control, particularly in slow flight when both wings and tail are generating maximum lift. The slender lifting surface model is very accurate at angles of attack below about 15°. At higher angles of attack vortex formation at the leading edge can stabilize the flow over the tail and thereby generate increased lift by a detached vortex mechanism. Asymmetry in the orientation of the leading edges with relation to the freestream (either in roll, yaw or caused by asymmetry in the planform) is amplified in the flow field and leads to large rolling and yawing forces that could be used for control in turning manoeuvres. The slender lifting surface model can be used to examine the effect of variations in tail shape and tail spread on the aerodynamic performance of the tail. A forked tail that has a triangular planform when spread to just over 120° gives the best aerodynamic performance and this may be close to a universal optimum, in terms of aerodynamic efficiency, for a means to control pitch and yaw. However, natural selection may act to optimise the performance of the tail when it is not widely spread. The tail is normally only widely spread during manoeuvres, or at low speeds, selection may act to improve the efficiency of the tail when it is spread to only a relatively narrow angle - for example to maximize the bird’s overall lift to drag ratio - the optimum shape at any angle of spread is that which gives a straight trailing edge to the tail. This will always give a slightly forked planform, but fork depth will depend on how widely the tail is spread when selection acts, and this depends on the criteria for optimization under natural selection. A forked tail is more sensitive to changes in angle of attack and angle of spread, than other tail types. Forked tails are more susceptible to damage than other tail morphologies, and suffer a greater loss of performance following damage. Forked tails also confer less inherent stability than any other type of tail. Aerodynamic performance may not be an im portant optimization criterion for birds that fly in a cluttered environment, or do not fly very much. Natural selection, under these conditions, may favour tails of other shapes. The aerodynamic costs of sexually selected elongated tails can be predicted from the model. These predictions can be used to distinguish between the various models for the evolution of elongated tails. Elongated graduated tails and pintails could have evolved either through a Fisherian or H andicap mechanism. The evolution of long forked tails can be initially favoured by natural selection, the pattern of feather elongation seen in sexually selected forked tails is predicted by the Fisher hypothesis (Fisher 1930) but not by any of the other theories of sexual selection.

@article{doi101098rstb19930079,
    author = "Thomas, Adrian L. R.",
    title = "On the aerodynamics of birds’ tails",
    year = "1993",
    journal = "Philosophical Transactions of the Royal Society B Biological Sciences",
    abstract = "Abstract The aerodynamic properties of a bird’s tail, and the forces produced by it, can be predicted by using slender lifting surface theory. The results of the model show that unlike conventional wings, which generate lift proportional to their area, the lift generated by the tail is proportional to the square of its maximum continuous span. Lift is unaffected by substantial variations in tail shape provided that the tail initially expands in width along the direction of flow. Behind the point of maximum width of the tail the flow is dominated by the wake of the forward section. Any area behind this point therefore causes only drag, not lift. The centre of lift is at the centre of area of the part of the tail in front of the point of maximum width. The moment arm of the tail, about its apex, is therefore more than twice the moment arm of a conventional wing about its leading edge. The drag of the tail is a combination of induced drag proportional to lift, and profile drag proportional to surface area. Induced drag can be halved by drooping the outer tail feathers to generate leading edge suction. This may be used for control, particularly in slow flight when both wings and tail are generating maximum lift. The slender lifting surface model is very accurate at angles of attack below about 15°. At higher angles of attack vortex formation at the leading edge can stabilize the flow over the tail and thereby generate increased lift by a detached vortex mechanism. Asymmetry in the orientation of the leading edges with relation to the freestream (either in roll, yaw or caused by asymmetry in the planform) is amplified in the flow field and leads to large rolling and yawing forces that could be used for control in turning manoeuvres. The slender lifting surface model can be used to examine the effect of variations in tail shape and tail spread on the aerodynamic performance of the tail. A forked tail that has a triangular planform when spread to just over 120° gives the best aerodynamic performance and this may be close to a universal optimum, in terms of aerodynamic efficiency, for a means to control pitch and yaw. However, natural selection may act to optimise the performance of the tail when it is not widely spread. The tail is normally only widely spread during manoeuvres, or at low speeds, selection may act to improve the efficiency of the tail when it is spread to only a relatively narrow angle - for example to maximize the bird’s overall lift to drag ratio - the optimum shape at any angle of spread is that which gives a straight trailing edge to the tail. This will always give a slightly forked planform, but fork depth will depend on how widely the tail is spread when selection acts, and this depends on the criteria for optimization under natural selection. A forked tail is more sensitive to changes in angle of attack and angle of spread, than other tail types. Forked tails are more susceptible to damage than other tail morphologies, and suffer a greater loss of performance following damage. Forked tails also confer less inherent stability than any other type of tail. Aerodynamic performance may not be an im portant optimization criterion for birds that fly in a cluttered environment, or do not fly very much. Natural selection, under these conditions, may favour tails of other shapes. The aerodynamic costs of sexually selected elongated tails can be predicted from the model. These predictions can be used to distinguish between the various models for the evolution of elongated tails. Elongated graduated tails and pintails could have evolved either through a Fisherian or H andicap mechanism. The evolution of long forked tails can be initially favoured by natural selection, the pattern of feather elongation seen in sexually selected forked tails is predicted by the Fisher hypothesis (Fisher 1930) but not by any of the other theories of sexual selection.",
    url = "https://doi.org/10.1098/rstb.1993.0079",
    doi = "10.1098/rstb.1993.0079",
    openalex = "W1995375509"
}

Abstract Birds migrating over land use either of two basic flight strategies, i.e. flapping or gliding/soaring flight. In soaring flight the birds gain altitude mainly by circling in thermals, i.e. rising air, and then they glide off until another thermal is encountered. Powered flapping flight is energetically much more expensive than gliding flight. This leaves us with the question why do not most birds adopt the soaring strategy rather than flapping flight on migration? I present optimization criteria, based on flight mechanical theory, for (i) energy-selected migration and (ii) time-selected migration, for flapping and soaring flight migration, respectively. These are evaluated in relation to general body size and rate of climb in thermals. I also consider the effects of wing morphology and horizontal winds. The general conclusion is that minimization of transport costs probably cannot be the only critical selective factor. In time-selected migration the size range of birds for which flapping flight is advantageous over thermal soaring flight, is significantly larger than in energy-selected migration, and this is in better agreement with what is found in real birds. Therefore, resulting migration speed probably constitutes an important selective force in bird migration. I also evaluate criteria for mixed strategies, i.e. when birds should use soaring flight when thermals are available and proceed by flapping flight otherwise. Finally, I also discuss some other factors, e.g. sensitivity to crosswinds, abundance of thermals and topography, which may affect the evolution of migration strategy.

@article{doi101098rstb19930164,
    author = "Hedenström, Anders",
    title = "Migration by soaring or flapping flight in birds: the relative importance of energy cost and speed",
    year = "1993",
    journal = "Philosophical Transactions of the Royal Society B Biological Sciences",
    abstract = "Abstract Birds migrating over land use either of two basic flight strategies, i.e. flapping or gliding/soaring flight. In soaring flight the birds gain altitude mainly by circling in thermals, i.e. rising air, and then they glide off until another thermal is encountered. Powered flapping flight is energetically much more expensive than gliding flight. This leaves us with the question why do not most birds adopt the soaring strategy rather than flapping flight on migration? I present optimization criteria, based on flight mechanical theory, for (i) energy-selected migration and (ii) time-selected migration, for flapping and soaring flight migration, respectively. These are evaluated in relation to general body size and rate of climb in thermals. I also consider the effects of wing morphology and horizontal winds. The general conclusion is that minimization of transport costs probably cannot be the only critical selective factor. In time-selected migration the size range of birds for which flapping flight is advantageous over thermal soaring flight, is significantly larger than in energy-selected migration, and this is in better agreement with what is found in real birds. Therefore, resulting migration speed probably constitutes an important selective force in bird migration. I also evaluate criteria for mixed strategies, i.e. when birds should use soaring flight when thermals are available and proceed by flapping flight otherwise. Finally, I also discuss some other factors, e.g. sensitivity to crosswinds, abundance of thermals and topography, which may affect the evolution of migration strategy.",
    url = "https://doi.org/10.1098/rstb.1993.0164",
    doi = "10.1098/rstb.1993.0164",
    openalex = "W1998937333",
    references = "tucker1970aerodynamics"
}

Abstract This is the first book on this subject since J W S Pringle's classic Insect Flight was published in 1957. Much has been written since on applied and ecological aspects of flight, but consideration of the question of the origin of wings and flight has been largely confined to armchair speculation in a scattered literature. To make matters worse, much of the recent empirical work has only appeared in Russain. A,K. Brodsky is a leading Russian authority on insect flight, a pioneer in the use of empirical aerodynamic techniques to unravel the mechanisms which underlie insect flight and hence its origins. By uniting fossil, structural, and phylogenetic information with his empirical studies, he draws a coherent, well-substantialed picture of the evolution of insect flight. The text is illustrated by numerous fine line drawings. The first book on insect flight since J.W.S. Pringle's classic Insect Flight was published in 1957 Written by leading Russian authority in the field - provides Western world with access to much previously unpublished material Draws togther the diverse strands of fossil evidence, structure, phylogeny, aerodynamics, and behaviour to form a coherent picture Beautifully illustrated with numerous fine line drawings

@book{doi101093oso97801985468180010001,
    author = "Brodsky, Andrei K",
    title = "The Evolution of Insect Flight",
    year = "1994",
    abstract = "Abstract This is the first book on this subject since J W S Pringle's classic Insect Flight was published in 1957. Much has been written since on applied and ecological aspects of flight, but consideration of the question of the origin of wings and flight has been largely confined to armchair speculation in a scattered literature. To make matters worse, much of the recent empirical work has only appeared in Russain. A,K. Brodsky is a leading Russian authority on insect flight, a pioneer in the use of empirical aerodynamic techniques to unravel the mechanisms which underlie insect flight and hence its origins. By uniting fossil, structural, and phylogenetic information with his empirical studies, he draws a coherent, well-substantialed picture of the evolution of insect flight. The text is illustrated by numerous fine line drawings. The first book on insect flight since J.W.S. Pringle's classic Insect Flight was published in 1957 Written by leading Russian authority in the field - provides Western world with access to much previously unpublished material Draws togther the diverse strands of fossil evidence, structure, phylogeny, aerodynamics, and behaviour to form a coherent picture Beautifully illustrated with numerous fine line drawings",
    url = "https://doi.org/10.1093/oso/9780198546818.001.0001",
    doi = "10.1093/oso/9780198546818.001.0001",
    openalex = "W4388245919"
}

Part I: Background for the Study of Vertebrate Anatomy. 1. Introduction. 2. Phylogenetic Relationships of Chordates and Craniates. 3. Diversity and Phylogenetic History of Craniates. 4. Early Development and Comparative Embryology. 5. Form and Function. Part II: Protection, Support, and Movement. 6. The Integument. 7. The Cranial Skeleton. 8. The Postcranial Skeleton: The Axial Skeleton. 9. The Postcranial Skeleton: The Appendicular Skeleton. 10. The Muscular System. 11. Functional Anatomy of Support and Locomotion. Part III: Integration. 12. The Sense Organs. 13. The Nervous System I: Organization, Spinal Cord, and Peripheral Nerves. 14. The Nervous System II: The Brain. 15. Endocrine Integration. Part IV: Metabolism and Reproduction. 16. The Digestive System: Oral Cavity and Feeding Mechanisms. 17. The Digestive System: Pharynx, Stomach, and Intestine. 18. The Respiratory System. 19. The Circulatory System. 20. The Excretory System and Osmoregulation. 21. The Reproductive System and Reproduction. Part V: Conclusion. 22. Conclusion/Epilogue.

@book{openalexw1546464865,
    author = "Liem, Karel F. and Bemis, William E. and Sillin, William B.",
    title = "Functional Anatomy of the Vertebrates: An Evolutionary Perspective",
    year = "1994",
    journal = "Medical Entomology and Zoology",
    abstract = "Part I: Background for the Study of Vertebrate Anatomy. 1. Introduction. 2. Phylogenetic Relationships of Chordates and Craniates. 3. Diversity and Phylogenetic History of Craniates. 4. Early Development and Comparative Embryology. 5. Form and Function. Part II: Protection, Support, and Movement. 6. The Integument. 7. The Cranial Skeleton. 8. The Postcranial Skeleton: The Axial Skeleton. 9. The Postcranial Skeleton: The Appendicular Skeleton. 10. The Muscular System. 11. Functional Anatomy of Support and Locomotion. Part III: Integration. 12. The Sense Organs. 13. The Nervous System I: Organization, Spinal Cord, and Peripheral Nerves. 14. The Nervous System II: The Brain. 15. Endocrine Integration. Part IV: Metabolism and Reproduction. 16. The Digestive System: Oral Cavity and Feeding Mechanisms. 17. The Digestive System: Pharynx, Stomach, and Intestine. 18. The Respiratory System. 19. The Circulatory System. 20. The Excretory System and Osmoregulation. 21. The Reproductive System and Reproduction. Part V: Conclusion. 22. Conclusion/Epilogue.",
    openalex = "W1546464865"
}

1. On the inference of function from structure George V. Lauder 2. The extant phylogenetic bracket and the importance of reconstructing soft tissues in fossils Lawrence M. Witmer 3. Fossils, function and phylogeny David B. Weishampel 4. Masticatory function in nonmammalian cynodonts and early mammals A. W. Crompton 5. Correlations between craniodental morphology and feeding behavior in ungulates: reciprocal illumination between living and fossil taxa Christine M. Janis 6. Functional predictions from theoretical models of the skull and jaws in reptiles and mammals Walter S. Greaves 7. Carnassial functioning in nimravid and felid sabretooths: theoretical basis and robustness of inferences Harold N. Bryant and Anthony P. Russell 8. The artificial determination of wear patterns on tooth models as a means to infer mandibular movement during feeding in mammals Virginia L. Naples 9. Determination of stresses in mammalian dental enamel and their relevance to the interpretation of feeding behaviors in extinct taxa John M. Rensberger 10. The structural consequences of skull flattening in crocodilians Arthur S. Busbey 11. Graphical analysis of dermal skull roof patterns Keith S. Thomson 12. The forelimb of Torosaurus, and an analysis of the posture and gait of ceratopsian dinosaurs Rolf E. Johnson and John H. Ostrom 13. Functional evolution of the hindlimb and tail from basal theropods to birds Stephen M. Gatesy 14. Functional interpretation of spinal anatomy in living and fossil amniotes Emily B. Giffin 15. To what extent may the mechanical environment of a bone be inferred from its internal architecture? Jeffrey J. Thomason 16. Form vs function: the evolution of a dialectic Kevin Padian.

@article{doi105860choice326223,
    title = "Functional morphology in vertebrate paleontology",
    year = "1995",
    journal = "Choice Reviews Online",
    abstract = "1. On the inference of function from structure George V. Lauder 2. The extant phylogenetic bracket and the importance of reconstructing soft tissues in fossils Lawrence M. Witmer 3. Fossils, function and phylogeny David B. Weishampel 4. Masticatory function in nonmammalian cynodonts and early mammals A. W. Crompton 5. Correlations between craniodental morphology and feeding behavior in ungulates: reciprocal illumination between living and fossil taxa Christine M. Janis 6. Functional predictions from theoretical models of the skull and jaws in reptiles and mammals Walter S. Greaves 7. Carnassial functioning in nimravid and felid sabretooths: theoretical basis and robustness of inferences Harold N. Bryant and Anthony P. Russell 8. The artificial determination of wear patterns on tooth models as a means to infer mandibular movement during feeding in mammals Virginia L. Naples 9. Determination of stresses in mammalian dental enamel and their relevance to the interpretation of feeding behaviors in extinct taxa John M. Rensberger 10. The structural consequences of skull flattening in crocodilians Arthur S. Busbey 11. Graphical analysis of dermal skull roof patterns Keith S. Thomson 12. The forelimb of Torosaurus, and an analysis of the posture and gait of ceratopsian dinosaurs Rolf E. Johnson and John H. Ostrom 13. Functional evolution of the hindlimb and tail from basal theropods to birds Stephen M. Gatesy 14. Functional interpretation of spinal anatomy in living and fossil amniotes Emily B. Giffin 15. To what extent may the mechanical environment of a bone be inferred from its internal architecture? Jeffrey J. Thomason 16. Form vs function: the evolution of a dialectic Kevin Padian.",
    url = "https://doi.org/10.5860/choice.32-6223",
    doi = "10.5860/choice.32-6223",
    openalex = "W3009764212"
}

The evolution of avian flight can be interpreted by analyzing the sequence of modifications of the primitive tetrapod locomotor system through time. Herein, we introduce the term "locomotor module" to identify anatomical subregions of the musculoskeletal system that are highly integrated and act as functional units during locomotion. The first tetrapods, which employed lateral undulations of the entire body and appendages, had one large locomotor module. Basal dinosaurs and theropods were bipedal and possessed a smaller locomotor module consisting of the hind limb and tail. Bird flight evolved as the superimposition of a second (aerial) locomotor capability onto the ancestral (terrestrial) theropod body plan. Although the origin of the wing module was the primary innovation, alterations in the terrestrial system were also significant. We propose that the primitive theropod locomotor module was functionally and anatomically subdivided into separate pelvic and caudal locomotor modules. This decoupling freed the tail to attain a new and intimate affiliation with the forelimb during flight, a configuration unique to birds. Thus, the evolution of flight can be viewed as the origin and novel association of locomotor modules. Differential elaboration of these modules in various lineages has produced the diverse locomotor abilities of modern birds.

@article{doi101111j155856461996tb04496x,
    author = "Gatesy, Stephen M. and Dial, Kenneth P.",
    title = "LOCOMOTOR MODULES AND THE EVOLUTION OF AVIAN FLIGHT",
    year = "1996",
    journal = "Evolution",
    abstract = {The evolution of avian flight can be interpreted by analyzing the sequence of modifications of the primitive tetrapod locomotor system through time. Herein, we introduce the term "locomotor module" to identify anatomical subregions of the musculoskeletal system that are highly integrated and act as functional units during locomotion. The first tetrapods, which employed lateral undulations of the entire body and appendages, had one large locomotor module. Basal dinosaurs and theropods were bipedal and possessed a smaller locomotor module consisting of the hind limb and tail. Bird flight evolved as the superimposition of a second (aerial) locomotor capability onto the ancestral (terrestrial) theropod body plan. Although the origin of the wing module was the primary innovation, alterations in the terrestrial system were also significant. We propose that the primitive theropod locomotor module was functionally and anatomically subdivided into separate pelvic and caudal locomotor modules. This decoupling freed the tail to attain a new and intimate affiliation with the forelimb during flight, a configuration unique to birds. Thus, the evolution of flight can be viewed as the origin and novel association of locomotor modules. Differential elaboration of these modules in various lineages has produced the diverse locomotor abilities of modern birds.},
    url = "https://doi.org/10.1111/j.1558-5646.1996.tb04496.x",
    doi = "10.1111/j.1558-5646.1996.tb04496.x",
    openalex = "W2314993465",
    references = "doi101016b9780122494055500094, doi101016s0016699588800664, doi101017s0094837300009866, doi101017s0094837300025495, doi101038361064a0, doi101038362623a0, doi10108002724634199410011523, doi1023073514548, doi105281zenodo16171435, doi105860choice326223, openalexw1504554173, openalexw2788234611"
}

This text is a comprehensive and illustrated discussion of the origin of and of avian flight. Ornithologist and evolutionary biologist Alan Feduccia, author of Age of Birds, here draws on fossil evidence and studies of the structure and biochemistry of living to present knowledge and data on avian evolution and propose a model of this evolutionary process. Feduccia begins with an overview of bird evolution, giving his opinions about the controversial problem in verte-brate paleontology: whether evolved directly from bipedal, terrestrial dinosaurs (the ground-up theory) or from the precursors of dinosaurs - perhaps small arboreal thecodonts (the trees-down theory). He then provides information about the origin of avian flight and feathers and discusses the most dramatic discoveries in avian paleontolgy of the past few decades - the opposite birds that were the dominant landbirds of the Mesozoic. Feduccia next offers a theory of avian evolution during the Tertiary, arguing that the evolution of follows a pattern similar to that of mammals, with an explosive (rather than gradual) evolution lasting only 5 to 10 million years. In the second half of the book he summarizes the evolution of all the modern orders of birds, discussing such subjects as the evolution of filter-feeding in ducks and flamingos, the evolution of flightlessness, the evolution of of prey and the rise of landbirds. The book also includes reconstructions of ancient fossil that have been prepared by bird artist John O'Neill.

@article{doi105860choice343307,
    title = "The origin and evolution of birds",
    year = "1997",
    journal = "Choice Reviews Online",
    abstract = "This text is a comprehensive and illustrated discussion of the origin of and of avian flight. Ornithologist and evolutionary biologist Alan Feduccia, author of Age of Birds, here draws on fossil evidence and studies of the structure and biochemistry of living to present knowledge and data on avian evolution and propose a model of this evolutionary process. Feduccia begins with an overview of bird evolution, giving his opinions about the controversial problem in verte-brate paleontology: whether evolved directly from bipedal, terrestrial dinosaurs (the ground-up theory) or from the precursors of dinosaurs - perhaps small arboreal thecodonts (the trees-down theory). He then provides information about the origin of avian flight and feathers and discusses the most dramatic discoveries in avian paleontolgy of the past few decades - the opposite birds that were the dominant landbirds of the Mesozoic. Feduccia next offers a theory of avian evolution during the Tertiary, arguing that the evolution of follows a pattern similar to that of mammals, with an explosive (rather than gradual) evolution lasting only 5 to 10 million years. In the second half of the book he summarizes the evolution of all the modern orders of birds, discussing such subjects as the evolution of filter-feeding in ducks and flamingos, the evolution of flightlessness, the evolution of of prey and the rise of landbirds. The book also includes reconstructions of ancient fossil that have been prepared by bird artist John O'Neill.",
    url = "https://doi.org/10.5860/choice.34-3307",
    doi = "10.5860/choice.34-3307",
    openalex = "W1555551215"
}

Introduction Part I Basic principles of insect flight Structure of the wing apparatus Mode of action of the wing apparatus The aerodynamics of insect flight Flight and behaviour Part II The evolution of insect flight The origins of flight and wings in insects Early forms of flight Flight based in hindwings From functionally four-winged to functionally two-winged flight Progress in insect flight Looking into the past: the process of evolution, and insect wing apparatus.

@book{openalexw1568848658,
    author = "Brodskiĭ, A. K.",
    title = "The Evolution of Insect Flight",
    year = "1997",
    journal = "Medical Entomology and Zoology",
    abstract = "Introduction Part I Basic principles of insect flight Structure of the wing apparatus Mode of action of the wing apparatus The aerodynamics of insect flight Flight and behaviour Part II The evolution of insect flight The origins of flight and wings in insects Early forms of flight Flight based in hindwings From functionally four-winged to functionally two-winged flight Progress in insect flight Looking into the past: the process of evolution, and insect wing apparatus.",
    openalex = "W1568848658"
}

ABSTRACT Birds evolved from and are phylogenetically recognized as members of the theropod dinosaurs; their first known member is the Late Jurassic Archaeopteryx, now represented by seven skeletons and a feather, and their closest known non‐avian relatives are the dromaeosaurid theropods such as Deinonychus. Bird flight is widely thought to have evolved from the trees down, but Archaeopteryx and its outgroups show no obvious arboreal or tree‐climbing characters, and its wing planform and wing loading do not resemble those of gliders. The ancestors of birds were bipedal, terrestrial, agile, cursorial and carnivorous or omnivorous. Apart from a perching foot and some skeletal fusions, a great many characters that are usually considered ‘avian’ (e.g. the furcula, the elongated forearm, the laterally flexing wrist and apparently feathers) evolved in non‐avian theropods for reasons unrelated to birds or to flight. Soon after Archaeopteryx, avian features such as the pygostyle, fusion of the carpometacarpus, and elongated curved pedal claws with a reversed, fully descended and opposable hallux, indicate improved flying ability and arboreal habits. In the further evolution of birds, characters related to the flight apparatus phylogenetically preceded those related to the rest of the skeleton and skull. Mesozoic birds are more diverse and numerous than thought previously and the most diverse known group of Cretaceous birds, the Enantiornithes, was not even recognized until 1981. The vast majority of Mesozoic bird groups have no Tertiary records: Enantiornithes, Hesperornithiformes, Ichthyornithiformes and several other lineages disappeared by the end of the Cretaceous. By that time, a few Linnean ‘Orders’ of extant birds had appeared, but none of these taxa belongs to extant ‘families’, and it is not until the Paleocene or (in most cases) the Eocene that the majority of extant bird ‘Orders’ are known in the fossil record. There is no evidence for a major or mass extinction of birds at the end of the Cretaceous, nor for a sudden ‘bottleneck’ in diversity that fostered the early Tertiary origination of living bird ‘Orders’.

@article{doi101111j1469185x1997tb00024x,
    author = "Padian, Kevin and Chiappe, Luis M.",
    title = "The origin and early evolution of birds",
    year = "1998",
    journal = "Biological reviews/Biological reviews of the Cambridge Philosophical Society",
    abstract = "ABSTRACT Birds evolved from and are phylogenetically recognized as members of the theropod dinosaurs; their first known member is the Late Jurassic Archaeopteryx, now represented by seven skeletons and a feather, and their closest known non‐avian relatives are the dromaeosaurid theropods such as Deinonychus. Bird flight is widely thought to have evolved from the trees down, but Archaeopteryx and its outgroups show no obvious arboreal or tree‐climbing characters, and its wing planform and wing loading do not resemble those of gliders. The ancestors of birds were bipedal, terrestrial, agile, cursorial and carnivorous or omnivorous. Apart from a perching foot and some skeletal fusions, a great many characters that are usually considered ‘avian’ (e.g. the furcula, the elongated forearm, the laterally flexing wrist and apparently feathers) evolved in non‐avian theropods for reasons unrelated to birds or to flight. Soon after Archaeopteryx, avian features such as the pygostyle, fusion of the carpometacarpus, and elongated curved pedal claws with a reversed, fully descended and opposable hallux, indicate improved flying ability and arboreal habits. In the further evolution of birds, characters related to the flight apparatus phylogenetically preceded those related to the rest of the skeleton and skull. Mesozoic birds are more diverse and numerous than thought previously and the most diverse known group of Cretaceous birds, the Enantiornithes, was not even recognized until 1981. The vast majority of Mesozoic bird groups have no Tertiary records: Enantiornithes, Hesperornithiformes, Ichthyornithiformes and several other lineages disappeared by the end of the Cretaceous. By that time, a few Linnean ‘Orders’ of extant birds had appeared, but none of these taxa belongs to extant ‘families’, and it is not until the Paleocene or (in most cases) the Eocene that the majority of extant bird ‘Orders’ are known in the fossil record. There is no evidence for a major or mass extinction of birds at the end of the Cretaceous, nor for a sudden ‘bottleneck’ in diversity that fostered the early Tertiary origination of living bird ‘Orders’.",
    url = "https://doi.org/10.1111/j.1469-185x.1997.tb00024.x",
    doi = "10.1111/j.1469-185x.1997.tb00024.x",
    openalex = "W2127438693",
    references = "doi101016b978012249408650011x, doi101016s0016699588800664, doi101016s0047248477800158, doi101038292051a0, doi101038331433a0, doi101038362623a0, doi101038378774a0, doi101038387390a0, doi101038nature01420, doi101038nature02706, doi101086407902, doi101098rstb19910056, doi101098rstb19920051, doi101111j109583121976tb00244x, doi101111j155856461996tb04496x, doi101126science1078237, doi101126science2555046845, doi101126science2665186779, doi101146annurevea03050175000415, doi1023071441916, doi1023073514548, doi102307jctt1xp3v3r, doi105281zenodo16171435, doi105860choice300927, doi105860choice343307, doi105860choice353642, houck1990allometric, openalexw1879660213, openalexw2991310333, openalexw3146596760, ostrom2019osteology"
}

Recent geophysical analyses suggest the presence of a late Paleozoic oxygen pulse beginning in the late Devonian and continuing through to the late Carboniferous. During this period, plant terrestrialization and global carbon deposition resulted in a dramatic increase in atmospheric oxygen levels, ultimately yielding concentrations potentially as high as 35% relative to the contemporary value of 21%. Such hyperoxia of the late Paleozoic atmosphere may have physiologically facilitated the initial evolution of insect flight metabolism. Widespread gigantism in late Paleozoic insects and other arthropods is also consistent with enhanced oxygen flux within diffusion-limited tracheal systems. Because total atmospheric pressure increases with increased oxygen partial pressure, concurrently hyperdense conditions would have augmented aerodynamic force production in early forms of flying insects. By the late Permian, evolution of decompositional microbial and fungal communities, together with disequilibrium in rates of carbon deposition, gradually reduced oxygen concentrations to values possibly as low as 15%. The disappearance of giant insects by the end of the Permian is consistent with extinction of these taxa for reasons of asphyxiation on a geological time scale. As with winged insects, the multiple historical origins of vertebrate flight in the late Jurassic and Cretaceous correlate temporally with periods of elevated atmospheric oxygen. Much discussion of flight performance in Archaeopteryx assumes a contemporary atmospheric composition. Elevated oxygen levels in the mid- to late Mesozoic would, however, have facilitated aerodynamic force production and enhanced muscle power output for ancestral birds, as well as for precursors to bats and pterosaurs.

@article{doi101146annurevphysiol621135,
    author = "Dudley, R",
    title = "The evolutionary physiology of animal flight: paleobiological and present perspectives.",
    year = "2000",
    journal = "Annual review of physiology",
    abstract = "Recent geophysical analyses suggest the presence of a late Paleozoic oxygen pulse beginning in the late Devonian and continuing through to the late Carboniferous. During this period, plant terrestrialization and global carbon deposition resulted in a dramatic increase in atmospheric oxygen levels, ultimately yielding concentrations potentially as high as 35\% relative to the contemporary value of 21\%. Such hyperoxia of the late Paleozoic atmosphere may have physiologically facilitated the initial evolution of insect flight metabolism. Widespread gigantism in late Paleozoic insects and other arthropods is also consistent with enhanced oxygen flux within diffusion-limited tracheal systems. Because total atmospheric pressure increases with increased oxygen partial pressure, concurrently hyperdense conditions would have augmented aerodynamic force production in early forms of flying insects. By the late Permian, evolution of decompositional microbial and fungal communities, together with disequilibrium in rates of carbon deposition, gradually reduced oxygen concentrations to values possibly as low as 15\%. The disappearance of giant insects by the end of the Permian is consistent with extinction of these taxa for reasons of asphyxiation on a geological time scale. As with winged insects, the multiple historical origins of vertebrate flight in the late Jurassic and Cretaceous correlate temporally with periods of elevated atmospheric oxygen. Much discussion of flight performance in Archaeopteryx assumes a contemporary atmospheric composition. Elevated oxygen levels in the mid- to late Mesozoic would, however, have facilitated aerodynamic force production and enhanced muscle power output for ancestral birds, as well as for precursors to bats and pterosaurs.",
    url = "https://pubmed.ncbi.nlm.nih.gov/10845087/",
    doi = "10.1146/annurev.physiol.62.1.135",
    openalex = "W2103355882",
    pmid = "10845087",
    references = "doi101016c20120016547, doi10103837918, doi101126science11536547, doi101126science27252651155, doi1015159780691220239, doi1023071483846, doi1023073514548, doi102475ajs294156, doi105860choice285664, doi105860choice324494"
}

Preface Acknowledgements List of main symbols List of figures List of tables 1. Introduction: a history of helicopter flight 2. Fundamentals of rotor aerodynamics 3. Blade element analysis 4. Rotating blade motion 5. Basic helicopter performance 6. Conceptual design of helicopters 7. Rotor airfoil aerodynamics 8. Unsteady aerodynamics 9. Dynamic stall 10. Rotor wakes and tip vortices Appendix Index.

@book{openalexw2103608534,
    author = "Leishman, J. Gordon",
    title = "Principles of Helicopter Aerodynamics",
    year = "2000",
    abstract = "Preface Acknowledgements List of main symbols List of figures List of tables 1. Introduction: a history of helicopter flight 2. Fundamentals of rotor aerodynamics 3. Blade element analysis 4. Rotating blade motion 5. Basic helicopter performance 6. Conceptual design of helicopters 7. Rotor airfoil aerodynamics 8. Unsteady aerodynamics 9. Dynamic stall 10. Rotor wakes and tip vortices Appendix Index.",
    url = "https://openalex.org/W2103608534",
    openalex = "W2103608534"
}

Low-speed aerodynamics is important in the design and operation of aircraft flying at low Mach number, and ground and marine vehicles. This 2001 book offers a modern treatment of the subject, both the theory of inviscid, incompressible, and irrotational aerodynamics and the computational techniques now available to solve complex problems. A unique feature of the text is that the computational approach (from a single vortex element to a three-dimensional panel formulation) is interwoven throughout. Thus, the reader can learn about classical methods of the past, while also learning how to use numerical methods to solve real-world aerodynamic problems. This second edition has a new chapter on the laminar boundary layer (emphasis on the viscous-inviscid coupling), the latest versions of computational techniques, and additional coverage of interaction problems. It includes a systematic treatment of two-dimensional panel methods and a detailed presentation of computational techniques for three-dimensional and unsteady flows. With extensive illustrations and examples, this book will be useful for senior and beginning graduate-level courses, as well as a helpful reference tool for practising engineers.

@book{doi101017cbo9780511810329,
    author = "Katz, Joseph and Plotkin, Allen",
    title = "Low-Speed Aerodynamics",
    year = "2001",
    booktitle = "Cambridge University Press eBooks",
    abstract = "Low-speed aerodynamics is important in the design and operation of aircraft flying at low Mach number, and ground and marine vehicles. This 2001 book offers a modern treatment of the subject, both the theory of inviscid, incompressible, and irrotational aerodynamics and the computational techniques now available to solve complex problems. A unique feature of the text is that the computational approach (from a single vortex element to a three-dimensional panel formulation) is interwoven throughout. Thus, the reader can learn about classical methods of the past, while also learning how to use numerical methods to solve real-world aerodynamic problems. This second edition has a new chapter on the laminar boundary layer (emphasis on the viscous-inviscid coupling), the latest versions of computational techniques, and additional coverage of interaction problems. It includes a systematic treatment of two-dimensional panel methods and a detailed presentation of computational techniques for three-dimensional and unsteady flows. With extensive illustrations and examples, this book will be useful for senior and beginning graduate-level courses, as well as a helpful reference tool for practising engineers.",
    url = "https://doi.org/10.1017/cbo9780511810329",
    doi = "10.1017/cbo9780511810329",
    openalex = "W62791640"
}

Femoral osteology and soft tissues evolved in a stepwise pattern in archosauromorph reptiles on the line to crown group birds. Crocodylia retains most ancestral archosaurian traits, whereas Dinosauromorpha (including birds) acquired many more derived traits. The complex sequence of changes included major shifts of several thigh muscle insertions. Medial rotation of the proximal femur (e.g. the femoral head) in archosaurs moved the greater trochanter laterally, bringing along the insertion of M. pubo‐ischio‐femoralis externus. Within Dinosauromorpha, the lesser trochanter moved proximally away from the trochanteric shelf. Presumably the lesser trochanter indicates the insertion of M. iliotrochantericus caudalis whereas the trochanteric shelf indicates the insertion of M. iliofemoralis externus. An accessory trochanter at the base of the lesser trochanter marks the insertion of M. pubo‐ischio‐femoralis internus 2 in tetanuran theropods. I propose hypotheses for the homologies of several intermuscular lines and other features on the femoral shaft. On the line to Neornithes, most changes of femoral morphology predated Aves and the origin of flight; few femoral features are unique to birds. Overall, the pattern of morphological evolution is consistent with stepwise functional evolution of the hindlimb within Dinosauromorpha on the line to Neornithes. The clade Ornithurae evolved the last few hindlimb apomorphies that characterize extant birds, in conjunction with more flexed hip and knee joints.

@article{doi101111j109636422001tb01314x,
    author = "Hutchinson, John R.",
    title = "The evolution of femoral osteology and soft tissues on the line to extant birds (Neornithes)",
    year = "2001",
    journal = "Zoological Journal of the Linnean Society",
    abstract = "Femoral osteology and soft tissues evolved in a stepwise pattern in archosauromorph reptiles on the line to crown group birds. Crocodylia retains most ancestral archosaurian traits, whereas Dinosauromorpha (including birds) acquired many more derived traits. The complex sequence of changes included major shifts of several thigh muscle insertions. Medial rotation of the proximal femur (e.g. the femoral head) in archosaurs moved the greater trochanter laterally, bringing along the insertion of M. pubo‐ischio‐femoralis externus. Within Dinosauromorpha, the lesser trochanter moved proximally away from the trochanteric shelf. Presumably the lesser trochanter indicates the insertion of M. iliotrochantericus caudalis whereas the trochanteric shelf indicates the insertion of M. iliofemoralis externus. An accessory trochanter at the base of the lesser trochanter marks the insertion of M. pubo‐ischio‐femoralis internus 2 in tetanuran theropods. I propose hypotheses for the homologies of several intermuscular lines and other features on the femoral shaft. On the line to Neornithes, most changes of femoral morphology predated Aves and the origin of flight; few femoral features are unique to birds. Overall, the pattern of morphological evolution is consistent with stepwise functional evolution of the hindlimb within Dinosauromorpha on the line to Neornithes. The clade Ornithurae evolved the last few hindlimb apomorphies that characterize extant birds, in conjunction with more flexed hip and knee joints.",
    url = "https://doi.org/10.1111/j.1096-3642.2001.tb01314.x",
    doi = "10.1111/j.1096-3642.2001.tb01314.x",
    openalex = "W2020878527",
    references = "coria1995a, doi101007bf02985709, doi101016b9781483231426500124, doi101017cbo9780511608377010, doi101017s0022336000026706, doi101017s0094837300009866, doi101038248168a0, doi101038292051a0, doi101038387390a0, doi10103845769, doi10108002724634199110011386, doi10108002724634199110011426, doi10108002724634199310011511, doi10108002724634199410011523, doi10108002724634199410011538, doi10108002724634199710011027, doi101086283367, doi101093sysbio33183, doi101098rstb19610007, doi101098rstb19650003, doi101098rstb19830079, doi101098rstb19850092, doi101098rstb19920117, doi101098rstb19990489, doi101111j109583121976tb00244x, doi101111j109600311988tb00514x, doi101111j109600311991tb00045x, doi101111j109600311995tb00092x, doi101111j109636422001tb01313x, doi101111j146979981991tb04794x, doi101111j155856461996tb04496x, doi101126science2555046845, doi101126science27853411267, doi101126science27953581915, doi101139e72031, doi101139e93179, doi1015468p4gnhz, doi1016660094837320000260734aaateo20co2, doi1023071292217, doi10230730135049, doi104095101672, doi105281zenodo1038220, doi105281zenodo16120887, doi105281zenodo16171435, doi105281zenodo16492064, doi105479si03629236110i, doi105860choice300927, doi105860choice326223, doi105860choice392183, doi105962p226819, gregor1988the, madsen1976a, openalexw2788234611, openalexw617951419, openalexw638862129, openalexw646636017, rowe1989a, walker1964triassic"
}

@article{doi101016s0169534702025685,
    author = "Hedenström, Anders",
    title = "Aerodynamics, evolution and ecology of avian flight",
    year = "2002",
    journal = "Trends in Ecology \& Evolution",
    url = "https://doi.org/10.1016/s0169-5347(02)02568-5",
    doi = "10.1016/s0169-5347(02)02568-5",
    openalex = "W1991260775"
}

@article{doi101016s0376042103000782,
    author = "Vermeer, L. J. and Sørensen, Jens Nørkær and Crespo, António",
    title = "Wind turbine wake aerodynamics",
    year = "2003",
    journal = "Progress in Aerospace Sciences",
    url = "https://doi.org/10.1016/s0376-0421(03)00078-2",
    doi = "10.1016/s0376-0421(03)00078-2",
    openalex = "W2162743401",
    references = "doi10100797836428401041, doi1010079783642858291, doi1010079783642859526, doi10100797836429148743, doi1010160045782574900292, doi101016s0142727x00000072, doi10111511471361, doi10251461978257, doi102514619932906, doi107551mitpress67810010001, openalexw3209569680"
}

Flapping wings of galliform birds are routinely used to produce aerodynamic forces oriented toward the substrate to enhance hindlimb traction. Here, I document this behavior in natural and laboratory settings. Adult birds fully capable of aerial flight preferentially employ wing-assisted incline running (WAIR), rather than flying, to reach elevated refuges (such as cliffs, trees, and boulders). From the day of hatching and before attaining sustained aerial flight, developing ground birds use WAIR to enhance their locomotor performance through improved foot traction, ultimately permitting vertical running. WAIR provides insight from behaviors observable in living birds into the possible role of incipient wings in feathered theropod dinosaurs and offers a previously unstudied explanation for the evolution of avian flight.

@article{doi101126science1078237,
    author = "Dial, Kenneth P.",
    title = "Wing-Assisted Incline Running and the Evolution of Flight",
    year = "2003",
    journal = "Science",
    abstract = "Flapping wings of galliform birds are routinely used to produce aerodynamic forces oriented toward the substrate to enhance hindlimb traction. Here, I document this behavior in natural and laboratory settings. Adult birds fully capable of aerial flight preferentially employ wing-assisted incline running (WAIR), rather than flying, to reach elevated refuges (such as cliffs, trees, and boulders). From the day of hatching and before attaining sustained aerial flight, developing ground birds use WAIR to enhance their locomotor performance through improved foot traction, ultimately permitting vertical running. WAIR provides insight from behaviors observable in living birds into the possible role of incipient wings in feathered theropod dinosaurs and offers a previously unstudied explanation for the evolution of avian flight.",
    url = "https://doi.org/10.1126/science.1078237",
    doi = "10.1126/science.1078237",
    openalex = "W1985053875",
    references = "doi10100797814615678751, doi1010160031018288901496, doi10103819967, doi10103831635, doi10103835017641, doi101086284076, doi101093oso97801951060840010001, doi101242jeb1992263, doi105860choice343307, openalexw23418293"
}

The flight of insects has fascinated physicists and biologists for more than a century. Yet, until recently, researchers were unable to rigorously quantify the complex wing motions of flapping insects or measure the forces and flows around their wings. However, recent developments in high-speed videography and tools for computational and mechanical modeling have allowed researchers to make rapid progress in advancing our understanding of insect flight. These mechanical and computational fluid dynamic models, combined with modern flow visualization techniques, have revealed that the fluid dynamic phenomena underlying flapping flight are different from those of non-flapping, 2-D wings on which most previous models were based. In particular, even at high angles of attack, a prominent leading edge vortex remains stably attached on the insect wing and does not shed into an unsteady wake, as would be expected from non-flapping 2-D wings. Its presence greatly enhances the forces generated by the wing, thus enabling insects to hover or maneuver. In addition, flight forces are further enhanced by other mechanisms acting during changes in angle of attack, especially at stroke reversal, the mutual interaction of the two wings at dorsal stroke reversal or wing-wake interactions following stroke reversal. This progress has enabled the development of simple analytical and empirical models that allow us to calculate the instantaneous forces on flapping insect wings more accurately than was previously possible. It also promises to foster new and exciting multi-disciplinary collaborations between physicists who seek to explain the phenomenology, biologists who seek to understand its relevance to insect physiology and evolution, and engineers who are inspired to build micro-robotic insects using these principles. This review covers the basic physical principles underlying flapping flight in insects, results of recent experiments concerning the aerodynamics of insect flight, as well as the different approaches used to model these phenomena.

@article{doi101242jeb00663,
    author = "Sane, Sanjay P.",
    title = "The aerodynamics of insect flight",
    year = "2003",
    journal = "Journal of Experimental Biology",
    abstract = "The flight of insects has fascinated physicists and biologists for more than a century. Yet, until recently, researchers were unable to rigorously quantify the complex wing motions of flapping insects or measure the forces and flows around their wings. However, recent developments in high-speed videography and tools for computational and mechanical modeling have allowed researchers to make rapid progress in advancing our understanding of insect flight. These mechanical and computational fluid dynamic models, combined with modern flow visualization techniques, have revealed that the fluid dynamic phenomena underlying flapping flight are different from those of non-flapping, 2-D wings on which most previous models were based. In particular, even at high angles of attack, a prominent leading edge vortex remains stably attached on the insect wing and does not shed into an unsteady wake, as would be expected from non-flapping 2-D wings. Its presence greatly enhances the forces generated by the wing, thus enabling insects to hover or maneuver. In addition, flight forces are further enhanced by other mechanisms acting during changes in angle of attack, especially at stroke reversal, the mutual interaction of the two wings at dorsal stroke reversal or wing-wake interactions following stroke reversal. This progress has enabled the development of simple analytical and empirical models that allow us to calculate the instantaneous forces on flapping insect wings more accurately than was previously possible. It also promises to foster new and exciting multi-disciplinary collaborations between physicists who seek to explain the phenomenology, biologists who seek to understand its relevance to insect physiology and evolution, and engineers who are inspired to build micro-robotic insects using these principles. This review covers the basic physical principles underlying flapping flight in insects, results of recent experiments concerning the aerodynamics of insect flight, as well as the different approaches used to model these phenomena.",
    url = "https://doi.org/10.1242/jeb.00663",
    doi = "10.1242/jeb.00663",
    openalex = "W2051788799",
    references = "doi1010079783642858291, doi101017cbo9780511800955, doi101038384626a0, doi101126science28454221954, doi10113719781611970517, doi101242jeb591169, doi1015159780691212975, doi1023072317984, ellington1984the, openalexw1518201591, openalexw1565704217"
}

Dental Functional Morphology offers an alternative to the received wisdom that teeth merely crush, cut, shear or grind food and shows how teeth adapt to diet. Providing an analysis of tooth action based on an understanding of how food particles break, it shows how tooth form from the earliest mammals to modern-day humans can be understood using very basic considerations about fracture. It outlines the theoretical basis step by step, explaining the factors governing tooth shape and size and provides an allometric analysis that will revolutionize attitudes to the evolution of the human face and the impact of cooked foods on our dentition. In addition, the basis of the mechanics behind the fracture of different types of food, and methods of measurement are given in an easy-to-use appendix. It will be an important sourcebook for physical anthropologists, dental and food scientists, palaeontologists and those interested in feeding ecology.

@book{doi101017cbo9780511735011,
    author = "Lucas, Peter W.",
    title = "Dental Functional Morphology",
    year = "2004",
    booktitle = "Cambridge University Press eBooks",
    abstract = "Dental Functional Morphology offers an alternative to the received wisdom that teeth merely crush, cut, shear or grind food and shows how teeth adapt to diet. Providing an analysis of tooth action based on an understanding of how food particles break, it shows how tooth form from the earliest mammals to modern-day humans can be understood using very basic considerations about fracture. It outlines the theoretical basis step by step, explaining the factors governing tooth shape and size and provides an allometric analysis that will revolutionize attitudes to the evolution of the human face and the impact of cooked foods on our dentition. In addition, the basis of the mechanics behind the fracture of different types of food, and methods of measurement are given in an easy-to-use appendix. It will be an important sourcebook for physical anthropologists, dental and food scientists, palaeontologists and those interested in feeding ecology.",
    url = "https://doi.org/10.1017/cbo9780511735011",
    doi = "10.1017/cbo9780511735011",
    openalex = "W4211168005",
    references = "doi101002ajpa1330550202, doi101002jmor1051850203, doi101006anbo20001261, doi101007bf00378906, doi101007s004250050096, doi101017cbo9780511565441, doi101017cbo9780511608551, doi101017cbo9780511623127, doi101017s0094837300006813, doi101017s0094837300013956, doi101017s1464793101005735, doi101038164820b0, doi101038246015a0, doi101038314260a0, doi101038363342a0, doi101038374798a0, doi101038416816a, doi101046j10958312200300146x, doi10108002724634199910011201, doi101086282934, doi101086284369, doi1010880508344333302, doi101098rsta19210006, doi101111j109636421952tb00784x, doi101111j109636421970tb00728x, doi101111j1365313x1993tb00007x, doi101111j1469185x1988tb00630x, doi101111j155856461947tb02711x, doi101111j155856461964tb01674x, doi101126science1864167892, doi101126science23547921038, doi101126science28454232137, doi101146annurevearth251435, doi101159000156428, doi1023072798801, doi1075919781501732355, openalexw1516188323, sereno1997the"
}

A biomechanically parsimonious hypothesis for the evolution of flapping flight in terrestrial vertebrates suggests progression within an arboreal context from jumping to directed aerial descent, gliding with control via appendicular motions, and ultimately to powered flight. The more than 30 phylogenetically independent lineages of arboreal vertebrate gliders lend strong indirect support to the ecological feasibility of such a trajectory. Insect flight evolution likely followed a similar sequence, but is unresolved paleontologically. Recently described falling behaviors in arboreal ants provide the first evidence demonstrating the biomechanical capacity for directed aerial descent in the complete absence of wings. Intentional control of body trajectories as animals fall from heights (and usually from vegetation) likely characterizes many more taxa than is currently recognized. Understanding the sensory and biomechanical mechanisms used by extant gliding animals to control and orient their descent is central to deciphering pathways involved in flight evolution.

@article{doi101146annurevecolsys37091305110014,
    author = "Dudley, Robert and Byrnes, Greg and Yanoviak, Stephen P. and Borrell, Brendan and Brown, Rafe M. and McGuire, Jimmy A.",
    title = "Gliding and the Functional Origins of Flight: Biomechanical Novelty or Necessity?",
    year = "2007",
    journal = "Annual Review of Ecology Evolution and Systematics",
    abstract = "A biomechanically parsimonious hypothesis for the evolution of flapping flight in terrestrial vertebrates suggests progression within an arboreal context from jumping to directed aerial descent, gliding with control via appendicular motions, and ultimately to powered flight. The more than 30 phylogenetically independent lineages of arboreal vertebrate gliders lend strong indirect support to the ecological feasibility of such a trajectory. Insect flight evolution likely followed a similar sequence, but is unresolved paleontologically. Recently described falling behaviors in arboreal ants provide the first evidence demonstrating the biomechanical capacity for directed aerial descent in the complete absence of wings. Intentional control of body trajectories as animals fall from heights (and usually from vegetation) likely characterizes many more taxa than is currently recognized. Understanding the sensory and biomechanical mechanisms used by extant gliding animals to control and orient their descent is central to deciphering pathways involved in flight evolution.",
    url = "https://doi.org/10.1146/annurev.ecolsys.37.091305.110014",
    doi = "10.1146/annurev.ecolsys.37.091305.110014",
    openalex = "W2108604151",
    references = "doi101073pnas0508724103, doi101126science1078237, doi101666040141"
}

SUMMARY Power output is a unifying theme for bird flight and considerable progress has been accomplished recently in measuring muscular, metabolic and aerodynamic power in birds. The primary flight muscles of birds, the pectoralis and supracoracoideus, are designed for work and power output, with large stress (force per unit cross-sectional area) and strain (relative length change) per contraction. U-shaped curves describe how mechanical power output varies with flight speed, but the specific shapes and characteristic speeds of these curves differ according to morphology and flight style. New measures of induced, profile and parasite power should help to update existing mathematical models of flight. In turn, these improved models may serve to test behavioral and ecological processes. Unlike terrestrial locomotion that is generally characterized by discrete gaits, changes in wing kinematics and aerodynamics across flight speeds are gradual. Take-off flight performance scales with body size, but fully revealing the mechanisms responsible for this pattern awaits new study. Intermittent flight appears to reduce the power cost for flight, as some species flap–glide at slow speeds and flap–bound at fast speeds. It is vital to test the metabolic costs of intermittent flight to understand why some birds use intermittent bounds during slow flight. Maneuvering and stability are critical for flying birds,and design for maneuvering may impinge upon other aspects of flight performance. The tail contributes to lift and drag; it is also integral to maneuvering and stability. Recent studies have revealed that maneuvers are typically initiated during downstroke and involve bilateral asymmetry of force production in the pectoralis. Future study of maneuvering and stability should measure inertial and aerodynamic forces. It is critical for continued progress into the biomechanics of bird flight that experimental designs are developed in an ecological and evolutionary context.

@article{doi101242jeb000273,
    author = "Tobalske, Bret W.",
    title = "Biomechanics of bird flight",
    year = "2007",
    journal = "Journal of Experimental Biology",
    abstract = "SUMMARY Power output is a unifying theme for bird flight and considerable progress has been accomplished recently in measuring muscular, metabolic and aerodynamic power in birds. The primary flight muscles of birds, the pectoralis and supracoracoideus, are designed for work and power output, with large stress (force per unit cross-sectional area) and strain (relative length change) per contraction. U-shaped curves describe how mechanical power output varies with flight speed, but the specific shapes and characteristic speeds of these curves differ according to morphology and flight style. New measures of induced, profile and parasite power should help to update existing mathematical models of flight. In turn, these improved models may serve to test behavioral and ecological processes. Unlike terrestrial locomotion that is generally characterized by discrete gaits, changes in wing kinematics and aerodynamics across flight speeds are gradual. Take-off flight performance scales with body size, but fully revealing the mechanisms responsible for this pattern awaits new study. Intermittent flight appears to reduce the power cost for flight, as some species flap–glide at slow speeds and flap–bound at fast speeds. It is vital to test the metabolic costs of intermittent flight to understand why some birds use intermittent bounds during slow flight. Maneuvering and stability are critical for flying birds,and design for maneuvering may impinge upon other aspects of flight performance. The tail contributes to lift and drag; it is also integral to maneuvering and stability. Recent studies have revealed that maneuvers are typically initiated during downstroke and involve bilateral asymmetry of force production in the pectoralis. Future study of maneuvering and stability should measure inertial and aerodynamic forces. It is critical for continued progress into the biomechanics of bird flight that experimental designs are developed in an ecological and evolutionary context.",
    url = "https://doi.org/10.1242/jeb.000273",
    doi = "10.1242/jeb.000273",
    openalex = "W2162782878",
    references = "doi101038nature01284"
}

Wing-assisted incline running (WAIR) is a form of locomotion in which a bird flaps its wings to aid its hindlimbs in climbing a slope. WAIR is used for escape in ground birds, and the ontogeny of this behavior in precocial birds has been suggested to represent a model analogous to transitional adaptive states during the evolution of powered avian flight. To begin to reveal the aerodynamics of flap-running, we used digital particle image velocimetry (DPIV) and measured air velocity, vorticity, circulation and added mass in the wake of chukar partridge Alectoris chukar as they engaged in WAIR (incline 65-85 degrees; N=7 birds) and ascending flight (85 degrees, N=2). To estimate lift and impulse, we coupled our DPIV data with three-dimensional wing kinematics from a companion study. The ontogeny of lift production was evaluated using three age classes: baby birds incapable of flight [6-8 days post hatching (d.p.h.)] and volant juveniles (25-28 days) and adults (45+ days). All three age classes of birds, including baby birds with partially emerged, symmetrical wing feathers, generated circulation with their wings and exhibited a wake structure that consisted of discrete vortex rings shed once per downstroke. Impulse of the vortex rings during WAIR was directed 45+/-5 degrees relative to horizontal and 21+/-4 degrees relative to the substrate. Absolute values of circulation in vortex cores and induced velocity increased with increasing age. Normalized circulation was similar among all ages in WAIR but 67% greater in adults during flight compared with flap-running. Estimated lift during WAIR was 6.6% of body weight in babies and between 63 and 86% of body weight in juveniles and adults. During flight, average lift was 110% of body weight. Our results reveal for the first time that lift from the wings, rather than wing inertia or profile drag, is primarily responsible for accelerating the body toward the substrate during WAIR, and that partially developed wings, not yet capable of flight, can produce useful lift during WAIR. We predict that neuromuscular control or power output, rather than external wing morphology, constrain the onset of flight ability during development in birds.

@article{doi101242jeb001701,
    author = "Tobalske, Bret W and Dial, Kenneth P",
    title = "Aerodynamics of wing-assisted incline running in birds.",
    year = "2007",
    journal = "The Journal of experimental biology",
    abstract = "Wing-assisted incline running (WAIR) is a form of locomotion in which a bird flaps its wings to aid its hindlimbs in climbing a slope. WAIR is used for escape in ground birds, and the ontogeny of this behavior in precocial birds has been suggested to represent a model analogous to transitional adaptive states during the evolution of powered avian flight. To begin to reveal the aerodynamics of flap-running, we used digital particle image velocimetry (DPIV) and measured air velocity, vorticity, circulation and added mass in the wake of chukar partridge Alectoris chukar as they engaged in WAIR (incline 65-85 degrees; N=7 birds) and ascending flight (85 degrees, N=2). To estimate lift and impulse, we coupled our DPIV data with three-dimensional wing kinematics from a companion study. The ontogeny of lift production was evaluated using three age classes: baby birds incapable of flight [6-8 days post hatching (d.p.h.)] and volant juveniles (25-28 days) and adults (45+ days). All three age classes of birds, including baby birds with partially emerged, symmetrical wing feathers, generated circulation with their wings and exhibited a wake structure that consisted of discrete vortex rings shed once per downstroke. Impulse of the vortex rings during WAIR was directed 45+/-5 degrees relative to horizontal and 21+/-4 degrees relative to the substrate. Absolute values of circulation in vortex cores and induced velocity increased with increasing age. Normalized circulation was similar among all ages in WAIR but 67\% greater in adults during flight compared with flap-running. Estimated lift during WAIR was 6.6\% of body weight in babies and between 63 and 86\% of body weight in juveniles and adults. During flight, average lift was 110\% of body weight. Our results reveal for the first time that lift from the wings, rather than wing inertia or profile drag, is primarily responsible for accelerating the body toward the substrate during WAIR, and that partially developed wings, not yet capable of flight, can produce useful lift during WAIR. We predict that neuromuscular control or power output, rather than external wing morphology, constrain the onset of flight ability during development in birds.",
    url = "https://pubmed.ncbi.nlm.nih.gov/17488937/",
    doi = "10.1242/jeb.001701",
    openalex = "W2159933015",
    pmid = "17488937",
    references = "doi101002sici1097010x199912152854291aidjez130co29, doi10100797814615678751, doi1010079783540723080, doi1010079783642838484, doi101016030096299090674h, doi101017cbo9780511800955, doi101017s0094837300004310, doi101038nature03647, doi10106312189885, doi101146annurevfl26010194003041"
}

Flight speed is expected to increase with mass and wing loading among flying animals and aircraft for fundamental aerodynamic reasons. Assuming geometrical and dynamical similarity, cruising flight speed is predicted to vary as (body mass)(1/6) and (wing loading)(1/2) among bird species. To test these scaling rules and the general importance of mass and wing loading for bird flight speeds, we used tracking radar to measure flapping flight speeds of individuals or flocks of migrating birds visually identified to species as well as their altitude and winds at the altitudes where the birds were flying. Equivalent airspeeds (airspeeds corrected to sea level air density, Ue) of 138 species, ranging 0.01-10 kg in mass, were analysed in relation to biometry and phylogeny. Scaling exponents in relation to mass and wing loading were significantly smaller than predicted (about 0.12 and 0.32, respectively, with similar results for analyses based on species and independent phylogenetic contrasts). These low scaling exponents may be the result of evolutionary restrictions on bird flight-speed range, counteracting too slow flight speeds among species with low wing loading and too fast speeds among species with high wing loading. This compression of speed range is partly attained through geometric differences, with aspect ratio showing a positive relationship with body mass and wing loading, but additional factors are required to fully explain the small scaling exponent of Ue in relation to wing loading. Furthermore, mass and wing loading accounted for only a limited proportion of the variation in Ue. Phylogeny was a powerful factor, in combination with wing loading, to account for the variation in Ue. These results demonstrate that functional flight adaptations and constraints associated with different evolutionary lineages have an important influence on cruising flapping flight speed that goes beyond the general aerodynamic scaling effects of mass and wing loading.

@article{doi101371journalpbio0050197,
    author = "Alerstam, Thomas and Rosén, Mikael and Bäckman, Johan and Ericson, Per G. P. and Hellgren, Olof",
    title = "Flight Speeds among Bird Species: Allometric and Phylogenetic Effects",
    year = "2007",
    journal = "PLoS Biology",
    abstract = "Flight speed is expected to increase with mass and wing loading among flying animals and aircraft for fundamental aerodynamic reasons. Assuming geometrical and dynamical similarity, cruising flight speed is predicted to vary as (body mass)(1/6) and (wing loading)(1/2) among bird species. To test these scaling rules and the general importance of mass and wing loading for bird flight speeds, we used tracking radar to measure flapping flight speeds of individuals or flocks of migrating birds visually identified to species as well as their altitude and winds at the altitudes where the birds were flying. Equivalent airspeeds (airspeeds corrected to sea level air density, Ue) of 138 species, ranging 0.01-10 kg in mass, were analysed in relation to biometry and phylogeny. Scaling exponents in relation to mass and wing loading were significantly smaller than predicted (about 0.12 and 0.32, respectively, with similar results for analyses based on species and independent phylogenetic contrasts). These low scaling exponents may be the result of evolutionary restrictions on bird flight-speed range, counteracting too slow flight speeds among species with low wing loading and too fast speeds among species with high wing loading. This compression of speed range is partly attained through geometric differences, with aspect ratio showing a positive relationship with body mass and wing loading, but additional factors are required to fully explain the small scaling exponent of Ue in relation to wing loading. Furthermore, mass and wing loading accounted for only a limited proportion of the variation in Ue. Phylogeny was a powerful factor, in combination with wing loading, to account for the variation in Ue. These results demonstrate that functional flight adaptations and constraints associated with different evolutionary lineages have an important influence on cruising flapping flight speed that goes beyond the general aerodynamic scaling effects of mass and wing loading.",
    url = "https://doi.org/10.1371/journal.pbio.0050197",
    doi = "10.1371/journal.pbio.0050197",
    openalex = "W2115605882",
    references = "doi101038nature01284"
}

@article{doi101007s0011400804883,
    author = "Hutchinson, John R. and Allen, Vivian",
    title = "The evolutionary continuum of limb function from early theropods to birds",
    year = "2008",
    journal = "Die Naturwissenschaften",
    url = "https://doi.org/10.1007/s00114-008-0488-3",
    doi = "10.1007/s00114-008-0488-3",
    openalex = "W2164139482",
    references = "carpenter2005the, coombs1980swimming, doi101002jmor10406, doi1010079789400904095, doi101016s002192900000155x, doi101016s0966636202000681, doi101038261129a0, doi1010384151018a, doi101073pnas0507106102, doi10108002724634199710010977, doi10108010635150802022231, doi101111j10963642200600245x, doi101111j14697580200600534x, doi101111j14754983200600585x, doi101111j15585646200700206x, doi101126science1078237, doi101126science2251499, doi101126science2740914, doi101126science28454232137, doi101130g23452a1, doi101144gslsp20042280106, doi101152jappl20008951991, doi10117702783649922066655, doi101242jeb001701, doi101242jeb005801, doi1016660094837320000260734aaateo20co2, doi1016660094837320050310676aohmma20co2, doi101666040141, doi105281zenodo16171435, doi105860choice326223, doi105860choice421568, doi105860choice434677"
}

@article{doi101038nature06517,
    author = "Dial, Kenneth P. and Jackson, Brandon E. and Segre, Paolo S.",
    title = "A fundamental avian wing-stroke provides a new perspective on the evolution of flight",
    year = "2008",
    journal = "Nature",
    url = "https://doi.org/10.1038/nature06517",
    doi = "10.1038/nature06517",
    openalex = "W2031748877",
    references = "doi101242jeb001701"
}

The study of morphological evolution after the inferred origin of active flight homologous with that in Aves has historically been characterized by an emphasis on anatomically disjunct, mosaic patterns of change. Relatively few prior studies have used discrete morphological character data in a phylogenetic context to quantitatively investigate morphological evolution or mosaic evolution in particular. One such previously employed method, which used summed unambiguously optimized synapomorphies, has been the basis for proposing disassociated and sequential "modernizing" or "fine-tuning" of pectoral and then pelvic locomotor systems after the origin of flight ("pectoral early-pelvic late" hypothesis). We use one of the most inclusive phylogenetic data sets of basal birds to investigate properties of this method and to consider the application of a Bayesian phylogenetic approach. Bayes factor and statistical comparisons of branch length estimates were used to evaluate support for a mosaic pattern of character change and the specific pectoral early-pelvic late hypothesis. Partitions were defined a priori based on anatomical subregion (e.g., pelvic, pectoral) and were based on those hypothesized using the summed synapomorphy approach. We compare 80 models all implementing the M(k) model for morphological data but varying in the number of anatomical subregion partitions, the models for among-partition rate variation and among-character rate variation, as well as the branch length prior. Statistical analysis reveals that partitioning data by anatomical subregion, independently estimating branch lengths for partitioned data, and use of shared or per partition gamma-shaped among-character rate distribution significantly increases estimated model likelihoods. Simulation studies reveal that partitioned models where characters are randomly assigned perform significantly worse than both the observed model and the single-partition equal-rate model, suggesting that only partitioning by anatomical subregion increases model performance. The preference for models with partitions defined a priori by anatomical subregion is consistent with a disjunctive pattern of character change for the data set investigated and may have implications for parameterization of Bayesian analyses of morphological data more generally. Statistical tests of differences in estimated branch lengths from the pectoral and pelvic partitions do not support the specific pectoral early-pelvic late hypothesis proposed from the summed synapomorphy approach; however, results suggest limited support for some pectoral branch lengths being significantly longer only early at/after the origin of flight.

@article{doi10108010635150802022231,
    author = "Clarke, Julia A. and Middleton, Kevin M.",
    title = "Mosaicism, Modules, and the Evolution of Birds: Results from a Bayesian Approach to the Study of Morphological Evolution Using Discrete Character Data",
    year = "2008",
    journal = "Systematic Biology",
    abstract = {The study of morphological evolution after the inferred origin of active flight homologous with that in Aves has historically been characterized by an emphasis on anatomically disjunct, mosaic patterns of change. Relatively few prior studies have used discrete morphological character data in a phylogenetic context to quantitatively investigate morphological evolution or mosaic evolution in particular. One such previously employed method, which used summed unambiguously optimized synapomorphies, has been the basis for proposing disassociated and sequential "modernizing" or "fine-tuning" of pectoral and then pelvic locomotor systems after the origin of flight ("pectoral early-pelvic late" hypothesis). We use one of the most inclusive phylogenetic data sets of basal birds to investigate properties of this method and to consider the application of a Bayesian phylogenetic approach. Bayes factor and statistical comparisons of branch length estimates were used to evaluate support for a mosaic pattern of character change and the specific pectoral early-pelvic late hypothesis. Partitions were defined a priori based on anatomical subregion (e.g., pelvic, pectoral) and were based on those hypothesized using the summed synapomorphy approach. We compare 80 models all implementing the M(k) model for morphological data but varying in the number of anatomical subregion partitions, the models for among-partition rate variation and among-character rate variation, as well as the branch length prior. Statistical analysis reveals that partitioning data by anatomical subregion, independently estimating branch lengths for partitioned data, and use of shared or per partition gamma-shaped among-character rate distribution significantly increases estimated model likelihoods. Simulation studies reveal that partitioned models where characters are randomly assigned perform significantly worse than both the observed model and the single-partition equal-rate model, suggesting that only partitioning by anatomical subregion increases model performance. The preference for models with partitions defined a priori by anatomical subregion is consistent with a disjunctive pattern of character change for the data set investigated and may have implications for parameterization of Bayesian analyses of morphological data more generally. Statistical tests of differences in estimated branch lengths from the pectoral and pelvic partitions do not support the specific pectoral early-pelvic late hypothesis proposed from the summed synapomorphy approach; however, results suggest limited support for some pectoral branch lengths being significantly longer only early at/after the origin of flight.},
    url = "https://doi.org/10.1080/10635150802022231",
    doi = "10.1080/10635150802022231",
    openalex = "W2118740788",
    references = "doi1010079780387217062, doi101007bf00160154, doi10108001621459199510476572, doi101080106351501753462876, doi10108010635150490264699, doi10108010635150600755396, doi101093bioinformatics178754, doi101093bioinformaticsbtg180, doi101111j14697580200600534x, doi101242jeb001701, doi102992014590582006371pon20co2, doi105860choice353862, doi105860choice392183, openalexw2994240441, openalexw3217097258"
}

Deep avian evolutionary relationships have been difficult to resolve as a result of a putative explosive radiation. Our study examined approximately 32 kilobases of aligned nuclear DNA sequences from 19 independent loci for 169 species, representing all major extant groups, and recovered a robust phylogeny from a genome-wide signal supported by multiple analytical methods. We documented well-supported, previously unrecognized interordinal relationships (such as a sister relationship between passerines and parrots) and corroborated previously contentious groupings (such as flamingos and grebes). Our conclusions challenge current classifications and alter our understanding of trait evolution; for example, some diurnal birds evolved from nocturnal ancestors. Our results provide a valuable resource for phylogenetic and comparative studies in birds.

@article{doi101126science1157704,
    author = "Hackett, Shannon J. and Kimball, Rebecca T. and Reddy, Sushma and Bowie, Rauri C. K. and Braun, Edward L. and Braun, Michael J. and Chojnowski, Jena L. and Cox, W. Andrew and Han, Kin-Lan and Harshman, John and Huddleston, Christopher J. and Marks, Ben D. and Miglia, Kathleen J. and Moore, William S. and Sheldon, Frederick H. and Steadman, David W. and Witt, Christopher C. and Yuri, Tamaki",
    title = "A Phylogenomic Study of Birds Reveals Their Evolutionary History",
    year = "2008",
    journal = "Science",
    abstract = "Deep avian evolutionary relationships have been difficult to resolve as a result of a putative explosive radiation. Our study examined approximately 32 kilobases of aligned nuclear DNA sequences from 19 independent loci for 169 species, representing all major extant groups, and recovered a robust phylogeny from a genome-wide signal supported by multiple analytical methods. We documented well-supported, previously unrecognized interordinal relationships (such as a sister relationship between passerines and parrots) and corroborated previously contentious groupings (such as flamingos and grebes). Our conclusions challenge current classifications and alter our understanding of trait evolution; for example, some diurnal birds evolved from nocturnal ancestors. Our results provide a valuable resource for phylogenetic and comparative studies in birds.",
    url = "https://doi.org/10.1126/science.1157704",
    doi = "10.1126/science.1157704",
    openalex = "W2107555182",
    references = "doi101006mpev19980603, doi101038nrg1603, doi101093bioinformaticsbtl446, doi101093sysbio422182, doi101098rsbl20060523, doi101111j109600312003tb00387x, doi101111j10963642200600293x, doi1023072992540, doi102307jctt1xp3v3r, doi105962bhltitle14581, openalexw1569611434"
}

This chapter contains sections titled: General Overview One-dimensional Momentum Theory and the Betz Limit Ideal Horizontal Axis Wind Turbine with Wake Rotation Airfoils and General Concepts of Aerodynamics Blade Design for Modern Wind Turbines Momentum Theory and Blade Element Theory Blade Shape for Ideal Rotor without Wake Rotation General Rotor Blade Shape Performance Prediction Blade Shape for Optimum Rotor with Wake Rotation Generalized Rotor Design Procedure Simplified HAWT Rotor Performance Calculation Procedure Effect of Drag and Blade Number on Optimum Performance Computational and Aerodynamic Issues in Aerodynamic Design Aerodynamics of Vertical Axis Wind Turbines References

@misc{doi1010029781119994367ch3,
    author = "Manwell, James F. and McGowan, J.G. and Rogers, Anthony",
    title = "Aerodynamics of Wind Turbines",
    year = "2009",
    abstract = "This chapter contains sections titled: General Overview One-dimensional Momentum Theory and the Betz Limit Ideal Horizontal Axis Wind Turbine with Wake Rotation Airfoils and General Concepts of Aerodynamics Blade Design for Modern Wind Turbines Momentum Theory and Blade Element Theory Blade Shape for Ideal Rotor without Wake Rotation General Rotor Blade Shape Performance Prediction Blade Shape for Optimum Rotor with Wake Rotation Generalized Rotor Design Procedure Simplified HAWT Rotor Performance Calculation Procedure Effect of Drag and Blade Number on Optimum Performance Computational and Aerodynamic Issues in Aerodynamic Design Aerodynamics of Vertical Axis Wind Turbines References",
    url = "https://doi.org/10.1002/9781119994367.ch3",
    doi = "10.1002/9781119994367.ch3",
    openalex = "W1945909317",
    references = "doi10100797836429148743, doi101016s0016003243912141, doi10111511626129, doi101146annurevfl25010193000555, doi101201b16587, openalexw1490468707, openalexw1497738216, openalexw1511449892, openalexw2894768479, openalexw569438991"
}

@article{doi101016jpaerosci201001001,
    author = "Shyy, W. and Aono, Hikaru and Chimakurthi, Satish Kumar and Trizila, Patrick Clark and Kang, Chang-Kwon and Cesnik, Carlos E. S. and Liu, Hao",
    title = "Recent progress in flapping wing aerodynamics and aeroelasticity",
    year = "2010",
    journal = "Progress in Aerospace Sciences",
    url = "https://doi.org/10.1016/j.paerosci.2010.01.001",
    doi = "10.1016/j.paerosci.2010.01.001",
    openalex = "W2067909757",
    references = "doi101016jjtbi200806011, doi101016jpaerosci200304001, doi101016jpaerosci200502001, doi101016jpaerosci200607001, doi101017cbo9780511551154, doi101017cbo9780511810329, doi101038384626a0, doi1010631857730, doi1010881748318233034001, doi101098rsif20090200, doi10111513162197, doi101126science28454221954, doi101146annurevfluid37061903175743, doi101242jeb00663, doi101242jeb01262, doi101242jeb1401137, doi101242jeb591169, doi102514152157, doi102514312149, ellington1984the, openalexw639581896"
}

TABLE OF CONTENTS Preface to the Fifth Edition Part 1: Fundamental Principles 1. Aerodynamics: Some Introductory Thoughts 2. Aerodynamics: Some Fundamental Principles and Equations Part 2: Inviscid, Incompressible Flow 3. Fundamentals of Inviscid, Incompressible Flow 4. Incompressible Flow Over Airfoils 5. Incompressible Flow Over Finite Wings 6. Three-Dimensional Incompressible Flow Part 3: Inviscid, Compressible Flow 7. Compressible Flow: Some Preliminary Aspects 8. Normal Shock Waves and Related Topics 9. Oblique Shock and Expansion Waves 10. Compressible Flow Through Nozzles, Diffusers and Wind Tunnels 11. Subsonic Compressible Flow Over Airfoils: Linear Theory 12. Linearized Supersonic Flow 13. Introduction to Numerical Techniques for Nonlinear Supersonic Flow 14. Elements of Hypersonic Flow Part 4: Viscous Flow 15. Introduction to the Fundamental Principles and Equations of Viscous Flow 16. A Special Case: Couette Flow 17. Introduction to Boundary Layers 18. Laminar Boundary Layers 19. Turbulent Boundary Layers 20. Navier-Stokes Solutions: Some Examples Appendix A: Isentropic Flow Properties Appendix B: Normal Shock Properties Appendix C: Prandtl-Meyer Function and Mach Angle Appendix D: Standard Atmosphere Bibliography Index

@article{doi102514152157,
    title = "Fundamentals of Aerodynamics",
    year = "2010",
    journal = "AIAA Journal",
    abstract = "TABLE OF CONTENTS Preface to the Fifth Edition Part 1: Fundamental Principles 1. Aerodynamics: Some Introductory Thoughts 2. Aerodynamics: Some Fundamental Principles and Equations Part 2: Inviscid, Incompressible Flow 3. Fundamentals of Inviscid, Incompressible Flow 4. Incompressible Flow Over Airfoils 5. Incompressible Flow Over Finite Wings 6. Three-Dimensional Incompressible Flow Part 3: Inviscid, Compressible Flow 7. Compressible Flow: Some Preliminary Aspects 8. Normal Shock Waves and Related Topics 9. Oblique Shock and Expansion Waves 10. Compressible Flow Through Nozzles, Diffusers and Wind Tunnels 11. Subsonic Compressible Flow Over Airfoils: Linear Theory 12. Linearized Supersonic Flow 13. Introduction to Numerical Techniques for Nonlinear Supersonic Flow 14. Elements of Hypersonic Flow Part 4: Viscous Flow 15. Introduction to the Fundamental Principles and Equations of Viscous Flow 16. A Special Case: Couette Flow 17. Introduction to Boundary Layers 18. Laminar Boundary Layers 19. Turbulent Boundary Layers 20. Navier-Stokes Solutions: Some Examples Appendix A: Isentropic Flow Properties Appendix B: Normal Shock Properties Appendix C: Prandtl-Meyer Function and Mach Angle Appendix D: Standard Atmosphere Bibliography Index",
    url = "https://doi.org/10.2514/152157",
    doi = "10.2514/152157",
    openalex = "W1549749298",
    references = "doi1010029781394309290ch3, doi1010160016003251905911, doi101016c20210017727, doi101017cbo9780511607158, doi10106313060977, doi101108eb029323, doi101177095441009721100102, openalexw1591096990, openalexw2997109191"
}

Saving energy and enhancing performance are secular preoccupations shared by both nature and human beings. In animal locomotion, flapping flyers or swimmers rely on the flexibility of their wings or body to passively increase their efficiency using an appropriate cycle of storing and releasing elastic energy. Despite the convergence of many observations pointing out this feature, the underlying mechanisms explaining how the elastic nature of the wings is related to propulsive efficiency remain unclear. Here we use an experiment with a self-propelled simplified insect model allowing to show how wing compliance governs the performance of flapping flyers. Reducing the description of the flapping wing to a forced oscillator model, we pinpoint different nonlinear effects that can account for the observed behavior--in particular a set of cubic nonlinearities coming from the clamped-free beam equation used to model the wing and a quadratic damping term representing the fluid drag associated to the fast flapping motion. In contrast to what has been repeatedly suggested in the literature, we show that flapping flyers optimize their performance not by especially looking for resonance to achieve larger flapping amplitudes with less effort, but by tuning the temporal evolution of the wing shape (i.e., the phase dynamics in the oscillator model) to optimize the aerodynamics.

@article{doi101073pnas1017910108,
    author = "Ramananarivo, Sophie and Godoy‐Diana, Ramiro and Thiria, Benjamin",
    title = "Rather than resonance, flapping wing flyers may play on aerodynamics to improve performance",
    year = "2011",
    journal = "Proceedings of the National Academy of Sciences",
    abstract = "Saving energy and enhancing performance are secular preoccupations shared by both nature and human beings. In animal locomotion, flapping flyers or swimmers rely on the flexibility of their wings or body to passively increase their efficiency using an appropriate cycle of storing and releasing elastic energy. Despite the convergence of many observations pointing out this feature, the underlying mechanisms explaining how the elastic nature of the wings is related to propulsive efficiency remain unclear. Here we use an experiment with a self-propelled simplified insect model allowing to show how wing compliance governs the performance of flapping flyers. Reducing the description of the flapping wing to a forced oscillator model, we pinpoint different nonlinear effects that can account for the observed behavior--in particular a set of cubic nonlinearities coming from the clamped-free beam equation used to model the wing and a quadratic damping term representing the fluid drag associated to the fast flapping motion. In contrast to what has been repeatedly suggested in the literature, we show that flapping flyers optimize their performance not by especially looking for resonance to achieve larger flapping amplitudes with less effort, but by tuning the temporal evolution of the wing shape (i.e., the phase dynamics in the oscillator model) to optimize the aerodynamics.",
    url = "https://doi.org/10.1073/pnas.1017910108",
    doi = "10.1073/pnas.1017910108",
    openalex = "W2059069846",
    references = "doi101016jpaerosci200304001, doi101016jpaerosci201001001, doi101017cbo9780511551154, doi101017s0022112097008392, doi101038nature02000, doi10111513153771, doi101126science28454221954, doi101146annurevfluid36050802121940, doi101242jeb200212705, doi1015159780691186344"
}

@article{doi101016jtree201112003,
    author = "Heers, Ashley M. and Dial, Kenneth P.",
    title = "From extant to extinct: locomotor ontogeny and the evolution of avian flight",
    year = "2012",
    journal = "Trends in Ecology \& Evolution",
    url = "https://doi.org/10.1016/j.tree.2011.12.003",
    doi = "10.1016/j.tree.2011.12.003",
    openalex = "W2135647030",
    references = "doi101007s0011400804883, doi101242jeb001701"
}

Recent discoveries of spectacular dinosaur fossils overwhelmingly support the hypothesis that birds are descended from maniraptoran theropod dinosaurs, and furthermore, demonstrate that distinctive bird characteristics such as feathers, flight, endothermic physiology, unique strategies for reproduction and growth, and a novel pulmonary system originated among Mesozoic terrestrial dinosaurs. The transition from ground-living to flight-capable theropod dinosaurs now probably represents one of the best-documented major evolutionary transitions in life history. Recent studies in developmental biology and other disciplines provide additional insights into how bird characteristics originated and evolved. The iconic features of extant birds for the most part evolved in a gradual and stepwise fashion throughout archosaur evolution. However, new data also highlight occasional bursts of morphological novelty at certain stages particularly close to the origin of birds and an unavoidable complex, mosaic evolutionary distribution of major bird characteristics on the theropod tree. Research into bird origins provides a premier example of how paleontological and neontological data can interact to reveal the complexity of major innovations, to answer key evolutionary questions, and to lead to new research directions. A better understanding of bird origins requires multifaceted and integrative approaches, yet fossils necessarily provide the final test of any evolutionary model.

@article{doi101126science1253293,
    author = "Xu, Xing and Zhou, Zhonghe and Dudley, Robert and Mackem, Susan and Chuong, Cheng‐Ming and Erickson, Gregory M. and Varricchio, David J.",
    title = "An integrative approach to understanding bird origins",
    year = "2014",
    journal = "Science",
    abstract = "Recent discoveries of spectacular dinosaur fossils overwhelmingly support the hypothesis that birds are descended from maniraptoran theropod dinosaurs, and furthermore, demonstrate that distinctive bird characteristics such as feathers, flight, endothermic physiology, unique strategies for reproduction and growth, and a novel pulmonary system originated among Mesozoic terrestrial dinosaurs. The transition from ground-living to flight-capable theropod dinosaurs now probably represents one of the best-documented major evolutionary transitions in life history. Recent studies in developmental biology and other disciplines provide additional insights into how bird characteristics originated and evolved. The iconic features of extant birds for the most part evolved in a gradual and stepwise fashion throughout archosaur evolution. However, new data also highlight occasional bursts of morphological novelty at certain stages particularly close to the origin of birds and an unavoidable complex, mosaic evolutionary distribution of major bird characteristics on the theropod tree. Research into bird origins provides a premier example of how paleontological and neontological data can interact to reveal the complexity of major innovations, to answer key evolutionary questions, and to lead to new research directions. A better understanding of bird origins requires multifaceted and integrative approaches, yet fossils necessarily provide the final test of any evolutionary model.",
    url = "https://doi.org/10.1126/science.1253293",
    doi = "10.1126/science.1253293",
    openalex = "W2033992760",
    references = "doi101002sici1097010x199912152854291aidjez130co29, doi10100797814615698316, doi101016jcub201209052, doi10103831635, doi10103834356, doi10103835086500, doi10103835086558, doi101038368196a0, doi101038378774a0, doi101038385247a0, doi101038387390a0, doi101038415780a, doi101038nature00930, doi101038nature01342, doi101038nature02898, doi101038nature03996, doi101038nature04511, doi101038nature05621, doi101038nature07447, doi101038nature07856, doi101038nature08322, doi101038nature08740, doi101038nature11146, doi101038nature12168, doi101038nature12424, doi101038nature12973, doi101038nature13467, doi101073pnas1203238109, doi10108002724634199910011125, doi101098rsbl20070254, doi101111j10963642200600245x, doi101111j1469185x201100190x, doi101111j155856461996tb04496x, doi101126science1078237, doi101126science1144066, doi101126science1180219, doi101126science1213780, doi101126science1225376, doi101126science1228753, doi101126science1252243, doi101126science1253143, doi101126science28454232137, doi101139e03011, doi1012067481, doi101371journalpone0003303, doi101371journalpone0007390, doi101371journalpone0036790, doi101371journalpone0081917, doi1015468gbdyof, doi105281zenodo16171435, doi105860choice421568, ostrom2019osteology"
}

This paper is the first part of the two-part exposition, addressing performance and dynamic stability of birds. The aerodynamic model underlying the entire study is presented in this part. It exploits the simplicity of the lifting line approximation to furnish the forces and moments acting on a single wing in closed analytical forms. The accuracy of the model is corroborated by comparison with numerical simulations based on the vortex lattice method. Performance is studied both in tethered (as on a sting in a wind tunnel) and in free flights. Wing twist is identified as the main parameter affecting the flight performance—at high speeds, it improves efficiency, the rate of climb and the maximal level speed; at low speeds, it allows flying slower. It is demonstrated that, under most circumstances, the difference in performance between tethered and free flights is small.

@article{iosilevskii2014forward,
    author = "Iosilevskii, G.",
    title = "Forward flight of birds revisited. Part 1: aerodynamics and performance",
    year = "2014",
    journal = "Royal Society Open Science",
    abstract = "This paper is the first part of the two-part exposition, addressing performance and dynamic stability of birds. The aerodynamic model underlying the entire study is presented in this part. It exploits the simplicity of the lifting line approximation to furnish the forces and moments acting on a single wing in closed analytical forms. The accuracy of the model is corroborated by comparison with numerical simulations based on the vortex lattice method. Performance is studied both in tethered (as on a sting in a wind tunnel) and in free flights. Wing twist is identified as the main parameter affecting the flight performance—at high speeds, it improves efficiency, the rate of climb and the maximal level speed; at low speeds, it allows flying slower. It is demonstrated that, under most circumstances, the difference in performance between tethered and free flights is small.",
    url = "https://doi.org/10.1098/rsos.140248",
    doi = "10.1098/rsos.140248",
    number = "2",
    openalex = "W2161665704",
    volume = "1",
    references = "doi1010160017931077900217, doi101017cbo9780511800955, doi101017cbo9780511810329, doi101017s0022112079000410, doi101038206226b0, doi101242jeb1992263, doi101242jeb19971613, doi101242jeb493527, doi102514313250, openalexw1587367332"
}

@article{doi101038nature15697,
    author = "Prum, Richard O. and Berv, Jacob S. and Dornburg, Alex and Field, Daniel J. and Townsend, Jeffrey P. and Lemmon, Emily Moriarty and Lemmon, Alan R.",
    title = "A comprehensive phylogeny of birds (Aves) using targeted next-generation DNA sequencing",
    year = "2015",
    journal = "Nature",
    url = "https://doi.org/10.1038/nature15697",
    doi = "10.1038/nature15697",
    openalex = "W1887892324",
    references = "doi101038nature11631, doi101073pnas0401892101, doi101073pnas1110395108, doi101093bioinformaticsbtq706, doi101093bioinformaticsbtu033, doi101093molbevmsl150, doi101093molbevmss020, doi101093molbevmss075, doi101093molbevmst010, doi101093sysbiosyr107, doi101098rspb20120683, doi101126science1157704, doi101126science1253451, doi101126science1257570, doi1012060003009020042860001mptaso20co2, doi101371journalpbio0040088"
}

The geometry of feather barbs (barb length and barb angle) determines feather vane asymmetry and vane rigidity, which are both critical to a feather's aerodynamic performance. Here, we describe the relationship between barb geometry and aerodynamic function across the evolutionary history of asymmetrical flight feathers, from Mesozoic taxa outside of modern avian diversity (Microraptor, Archaeopteryx, Sapeornis, Confuciusornis and the enantiornithine Eopengornis) to an extensive sample of modern birds. Contrary to previous assumptions, we find that barb angle is not related to vane-width asymmetry; instead barb angle varies with vane function, whereas barb length variation determines vane asymmetry. We demonstrate that barb geometry significantly differs among functionally distinct portions of flight feather vanes, and that cutting-edge leading vanes occupy a distinct region of morphospace characterized by small barb angles. This cutting-edge vane morphology is ubiquitous across a phylogenetically and functionally diverse sample of modern birds and Mesozoic stem birds, revealing a fundamental aerodynamic adaptation that has persisted from the Late Jurassic. However, in Mesozoic taxa stemward of Ornithurae and Enantiornithes, trailing vane barb geometry is distinctly different from that of modern birds. In both modern birds and enantiornithines, trailing vanes have larger barb angles than in comparatively stemward taxa like Archaeopteryx, which exhibit small trailing vane barb angles. This discovery reveals a previously unrecognized evolutionary transition in flight feather morphology, which has important implications for the flight capacity of early feathered theropods such as Archaeopteryx and Microraptor. Our findings suggest that the fully modern avian flight feather, and possibly a modern capacity for powered flight, evolved crownward of Confuciusornis, long after the origin of asymmetrical flight feathers, and much later than previously recognized.

@article{doi101098rspb20142864,
    author = "Feo, Teresa J. and Field, Daniel J. and Prum, Richard O.",
    title = "Barb geometry of asymmetrical feathers reveals a transitional morphology in the evolution of avian flight",
    year = "2015",
    journal = "Proceedings of the Royal Society B Biological Sciences",
    abstract = "The geometry of feather barbs (barb length and barb angle) determines feather vane asymmetry and vane rigidity, which are both critical to a feather's aerodynamic performance. Here, we describe the relationship between barb geometry and aerodynamic function across the evolutionary history of asymmetrical flight feathers, from Mesozoic taxa outside of modern avian diversity (Microraptor, Archaeopteryx, Sapeornis, Confuciusornis and the enantiornithine Eopengornis) to an extensive sample of modern birds. Contrary to previous assumptions, we find that barb angle is not related to vane-width asymmetry; instead barb angle varies with vane function, whereas barb length variation determines vane asymmetry. We demonstrate that barb geometry significantly differs among functionally distinct portions of flight feather vanes, and that cutting-edge leading vanes occupy a distinct region of morphospace characterized by small barb angles. This cutting-edge vane morphology is ubiquitous across a phylogenetically and functionally diverse sample of modern birds and Mesozoic stem birds, revealing a fundamental aerodynamic adaptation that has persisted from the Late Jurassic. However, in Mesozoic taxa stemward of Ornithurae and Enantiornithes, trailing vane barb geometry is distinctly different from that of modern birds. In both modern birds and enantiornithines, trailing vanes have larger barb angles than in comparatively stemward taxa like Archaeopteryx, which exhibit small trailing vane barb angles. This discovery reveals a previously unrecognized evolutionary transition in flight feather morphology, which has important implications for the flight capacity of early feathered theropods such as Archaeopteryx and Microraptor. Our findings suggest that the fully modern avian flight feather, and possibly a modern capacity for powered flight, evolved crownward of Confuciusornis, long after the origin of asymmetrical flight feathers, and much later than previously recognized.",
    url = "https://doi.org/10.1098/rspb.2014.2864",
    doi = "10.1098/rspb.2014.2864",
    openalex = "W1986640165",
    references = "doi101038nature13467, doi101371journalpone0082000, doi101666040141"
}

Bird flight is a remarkable adaptation that has allowed the approximately 10 000 extant species to colonize all terrestrial habitats on earth including high elevations, polar regions, distant islands, arid deserts, and many others. Birds exhibit numerous physiological and biomechanical adaptations for flight. Although bird flight is often studied at the level of aerodynamics, morphology, wingbeat kinematics, muscle activity, or sensory guidance independently, in reality these systems are naturally integrated. There has been an abundance of new studies in these mechanistic aspects of avian biology but comparatively less recent work on the physiological ecology of avian flight. Here we review research at the interface of the systems used in flight control and discuss several common themes. Modulation of aerodynamic forces to respond to different challenges is driven by three primary mechanisms: wing velocity about the shoulder, shape within the wing, and angle of attack. For birds that flap, the distinction between velocity and shape modulation synthesizes diverse studies in morphology, wing motion, and motor control. Recently developed tools for studying bird flight are influencing multiple areas of investigation, and in particular the role of sensory systems in flight control. How sensory information is transformed into motor commands in the avian brain remains, however, a largely unexplored frontier.

@article{doi101139cjz20150103,
    author = "Altshuler, Douglas L. and Bahlman, Joseph W. and Dakin, Roslyn and Gaede, Andrea H. and Goller, Benjamin and Lentink, David and Segre, Paolo S. and Skandalis, Dimitri A.",
    title = "The biophysics of bird flight: functional relationships integrate aerodynamics, morphology, kinematics, muscles, and sensors",
    year = "2015",
    journal = "Canadian Journal of Zoology",
    abstract = "Bird flight is a remarkable adaptation that has allowed the approximately 10 000 extant species to colonize all terrestrial habitats on earth including high elevations, polar regions, distant islands, arid deserts, and many others. Birds exhibit numerous physiological and biomechanical adaptations for flight. Although bird flight is often studied at the level of aerodynamics, morphology, wingbeat kinematics, muscle activity, or sensory guidance independently, in reality these systems are naturally integrated. There has been an abundance of new studies in these mechanistic aspects of avian biology but comparatively less recent work on the physiological ecology of avian flight. Here we review research at the interface of the systems used in flight control and discuss several common themes. Modulation of aerodynamic forces to respond to different challenges is driven by three primary mechanisms: wing velocity about the shoulder, shape within the wing, and angle of attack. For birds that flap, the distinction between velocity and shape modulation synthesizes diverse studies in morphology, wing motion, and motor control. Recently developed tools for studying bird flight are influencing multiple areas of investigation, and in particular the role of sensory systems in flight control. How sensory information is transformed into motor commands in the avian brain remains, however, a largely unexplored frontier.",
    url = "https://doi.org/10.1139/cjz-2015-0103",
    doi = "10.1139/cjz-2015-0103",
    openalex = "W1925405177",
    references = "doi101001archopht195201700030123016, doi101007s1033600702136, doi101007s1231101103319, doi101016b9780122494055500094, doi101038293293a0, doi101038384626a0, doi1010881748318233034001, doi101126science176403062, doi101126science27553031113, doi101126science28454221954, doi101242jeb591169"
}

Aerodynamics of Wind Turbines is the established essential text for the fundamental solutions to efficient wind turbine design. Now in its third edition, it has been substantially updated with respect to structural dynamics and control. The new control chapter now includes details on how to design a classical pitch and torque regulator to control rotational speed and power, while the section on structural dynamics has been extended with a simplified mechanical system explaining the phenomena of forward and backward whirling modes. Readers will also benefit from a new chapter on Vertical Axis W

@book{doi1043249781315769981,
    author = "Hansen, Martin Otto Lavér",
    title = "Aerodynamics of Wind Turbines",
    year = "2015",
    abstract = "Aerodynamics of Wind Turbines is the established essential text for the fundamental solutions to efficient wind turbine design. Now in its third edition, it has been substantially updated with respect to structural dynamics and control. The new control chapter now includes details on how to design a classical pitch and torque regulator to control rotational speed and power, while the section on structural dynamics has been extended with a simplified mechanical system explaining the phenomena of forward and backward whirling modes. Readers will also benefit from a new chapter on Vertical Axis W",
    url = "https://doi.org/10.4324/9781315769981",
    doi = "10.4324/9781315769981",
    openalex = "W219929715"
}

@article{doi101038nature19417,
    author = "Prum, Richard O. and Berv, Jacob S. and Dornburg, Alex and Field, Daniel J. and Townsend, Jeffrey P. and Lemmon, Emily Moriarty and Lemmon, Alan R.",
    title = "A comprehensive phylogeny of birds (Aves) using targeted next-generation DNA sequencing",
    year = "2016",
    journal = "Nature",
    url = "https://doi.org/10.1038/nature19417",
    doi = "10.1038/nature19417",
    openalex = "W4231969088",
    references = "doi105860choice405235"
}

A firm understanding of how fruit flies hover has emerged over the past two decades, and recent work has focused on the aerodynamic, biomechanical and neurobiological mechanisms that enable them to manoeuvre and resist perturbations. In this review, we describe how flies manipulate wing movement to control their body motion during active manoeuvres, and how these actions are regulated by sensory feedback. We also discuss how the application of control theory is providing new insight into the logic and structure of the circuitry that underlies flight stability.This article is part of the themed issue 'Moving in a moving medium: new perspectives on flight'.

@article{doi101098rstb20150388,
    author = "Dickinson, Michael H. and Muijres, Florian T.",
    title = "The aerodynamics and control of free flight manoeuvres in Drosophila",
    year = "2016",
    journal = "Philosophical Transactions of the Royal Society B Biological Sciences",
    abstract = "A firm understanding of how fruit flies hover has emerged over the past two decades, and recent work has focused on the aerodynamic, biomechanical and neurobiological mechanisms that enable them to manoeuvre and resist perturbations. In this review, we describe how flies manipulate wing movement to control their body motion during active manoeuvres, and how these actions are regulated by sensory feedback. We also discuss how the application of control theory is providing new insight into the logic and structure of the circuitry that underlies flight stability.This article is part of the themed issue 'Moving in a moving medium: new perspectives on flight'.",
    url = "https://doi.org/10.1098/rstb.2015.0388",
    doi = "10.1098/rstb.2015.0388",
    openalex = "W2507672249",
    references = "doi101007bf00622503, doi101038384626a0, doi101038nbt3439, doi101093oso97801950623970010001, doi101098rstb19840051, doi101103physrevlett104148101, doi101126science1258096, doi101126science28454221954, doi101126science2885463100, doi105860choice320998, doi105860choice462107"
}

More than a million insects and approximately 11,000 vertebrates utilize flapping wings to fly. However, flapping flight has only been studied in a few of these species, so many challenges remain in understanding this form of locomotion. Five key aerodynamic mechanisms have been identified for insect flight. Among these is the leading edge vortex, which is a convergent solution to avoid stall for insects, bats and birds. The roles of the other mechanisms - added mass, clap and fling, rotational circulation and wing-wake interactions - have not yet been thoroughly studied in the context of vertebrate flight. Further challenges to understanding bat and bird flight are posed by the complex, dynamic wing morphologies of these species and the more turbulent airflow generated by their wings compared with that observed during insect flight. Nevertheless, three dimensionless numbers that combine key flow, morphological and kinematic parameters - the Reynolds number, Rossby number and advance ratio - govern flapping wing aerodynamics for both insects and vertebrates. These numbers can thus be used to organize an integrative framework for studying and comparing animal flapping flight. Here, we provide a roadmap for developing such a framework, highlighting the aerodynamic mechanisms that remain to be quantified and compared across species. Ultimately, incorporating complex flight maneuvers, environmental effects and developmental stages into this framework will also be essential to advancing our understanding of the biomechanics, movement ecology and evolution of animal flight.

@article{doi101242jeb042317,
    author = "Chin, Diana D. and Lentink, David",
    title = "Flapping wing aerodynamics: from insects to vertebrates",
    year = "2016",
    journal = "Journal of Experimental Biology",
    abstract = "More than a million insects and approximately 11,000 vertebrates utilize flapping wings to fly. However, flapping flight has only been studied in a few of these species, so many challenges remain in understanding this form of locomotion. Five key aerodynamic mechanisms have been identified for insect flight. Among these is the leading edge vortex, which is a convergent solution to avoid stall for insects, bats and birds. The roles of the other mechanisms - added mass, clap and fling, rotational circulation and wing-wake interactions - have not yet been thoroughly studied in the context of vertebrate flight. Further challenges to understanding bat and bird flight are posed by the complex, dynamic wing morphologies of these species and the more turbulent airflow generated by their wings compared with that observed during insect flight. Nevertheless, three dimensionless numbers that combine key flow, morphological and kinematic parameters - the Reynolds number, Rossby number and advance ratio - govern flapping wing aerodynamics for both insects and vertebrates. These numbers can thus be used to organize an integrative framework for studying and comparing animal flapping flight. Here, we provide a roadmap for developing such a framework, highlighting the aerodynamic mechanisms that remain to be quantified and compared across species. Ultimately, incorporating complex flight maneuvers, environmental effects and developmental stages into this framework will also be essential to advancing our understanding of the biomechanics, movement ecology and evolution of animal flight.",
    url = "https://doi.org/10.1242/jeb.042317",
    doi = "10.1242/jeb.042317",
    openalex = "W2335421812",
    references = "doi10100797814615678751, doi101098rsif20090200, doi101111evo12681, doi101242jeb00663"
}

Using our first principles approach we find that "near flight" locomotor behaviors are most sensitive to wing area, and that non-locomotory related selection regimes likely expanded wing area well before WAIR and other such behaviors were possible in derived avians. These results suggest that investigations of the drivers for wing expansion and feather elongation in theropods need not be intrinsically linked to locomotory adaptations, and this separation is critical for our understanding of the origin of powered flight and avian evolution.

@article{doi107717peerj2159,
    author = "Dececchi, T. Alexander and Larsson, Hans C. E. and Habib, Michael",
    title = "The wings before the bird: an evaluation of flapping-based locomotory hypotheses in bird antecedents",
    year = "2016",
    journal = "PeerJ",
    abstract = {Using our first principles approach we find that "near flight" locomotor behaviors are most sensitive to wing area, and that non-locomotory related selection regimes likely expanded wing area well before WAIR and other such behaviors were possible in derived avians. These results suggest that investigations of the drivers for wing expansion and feather elongation in theropods need not be intrinsically linked to locomotory adaptations, and this separation is critical for our understanding of the origin of powered flight and avian evolution.},
    url = "https://doi.org/10.7717/peerj.2159",
    doi = "10.7717/peerj.2159",
    openalex = "W2273958615",
    references = "crossref2007basic, doi101016jcub201209052, doi101016s1875306x08x00024, doi101038114085a0, doi10103831635, doi10103835047056, doi101038nature01342, doi101038nature11631, doi101093aesa9351195f, doi101111j146979981991tb04794x, doi101126science1225376, doi101242jeb001701, doi101242jeb1301235, doi101371journalpone0028964, doi101371journalpone0121476, doi1017161paleo180818764, iosilevskii2014forward, openalexw2240950819"
}

Flight is the defining characteristic of birds, yet the mechanisms through which flight ability develops are only beginning to be understood. Wing-assisted incline running (WAIR) and controlled flapping descent (CFD) are behaviors that may offer significant adaptive benefits to developing birds. Recent research into these forms of locomotion has focused on species with precocial development, with a particularly rich data set from chukar partridge (Alectoris chukar). Here we briefly review the kinematics and aerodynamics of flight development in this species. We then present novel measurements of the development of pectoralis contractile behavior during the ontogenetic transition toward powered flight. To obtain these new empirical data, we used indwelling electromyography (EMG) and sonomicrometry and tested WAIR and CFD in seven age classes of chukar (n = 2-4 birds per age) from 5 days post hatching (dph) to adult (300+ dph). For each age class, we measured muscle activity during maximal performance, which was WAIR at 65° in birds 5 dph, CFD in birds 9 dph, WAIR at 80° in birds 14 dph, level flight in birds 25-61 dph, and ascending flight in adults. We also measured muscle activity during sub-maximal performance in all age classes. Flapping chukar chicks use near-continuous activation of their pectoralis at relatively low electromyography amplitudes for the first 8 days and progress to stereotypic higher-amplitude activation bursts by Day 12. The pectoralis undergoes increasing strain at higher strain rates with age, and length trajectory becomes more asymmetrical with greater variation in contractile velocity within the shortening phase of individual contractions. At 20-25 days (12-15% adult chukar mass), pectoralis activity and locomotor performance approaches that of adults, although strain rate exhibits a temporary decrease at 61 dph concurrent with using newly-replaced primary feathers. To better understand how these patterns relate to the evolution of life-history strategy and locomotion, we encourage future efforts to explore these behaviors in altricial and semi-altricial bird species.

@article{doi101093icbicx050,
    author = "Tobalske, Bret W and Jackson, Brandon E and Dial, Kenneth P",
    title = "Ontogeny of Flight Capacity and Pectoralis Function in a Precocial Ground Bird (Alectoris chukar).",
    year = "2017",
    journal = "Integrative and comparative biology",
    abstract = "Flight is the defining characteristic of birds, yet the mechanisms through which flight ability develops are only beginning to be understood. Wing-assisted incline running (WAIR) and controlled flapping descent (CFD) are behaviors that may offer significant adaptive benefits to developing birds. Recent research into these forms of locomotion has focused on species with precocial development, with a particularly rich data set from chukar partridge (Alectoris chukar). Here we briefly review the kinematics and aerodynamics of flight development in this species. We then present novel measurements of the development of pectoralis contractile behavior during the ontogenetic transition toward powered flight. To obtain these new empirical data, we used indwelling electromyography (EMG) and sonomicrometry and tested WAIR and CFD in seven age classes of chukar (n = 2-4 birds per age) from 5 days post hatching (dph) to adult (300+ dph). For each age class, we measured muscle activity during maximal performance, which was WAIR at 65° in birds 5 dph, CFD in birds 9 dph, WAIR at 80° in birds 14 dph, level flight in birds 25-61 dph, and ascending flight in adults. We also measured muscle activity during sub-maximal performance in all age classes. Flapping chukar chicks use near-continuous activation of their pectoralis at relatively low electromyography amplitudes for the first 8 days and progress to stereotypic higher-amplitude activation bursts by Day 12. The pectoralis undergoes increasing strain at higher strain rates with age, and length trajectory becomes more asymmetrical with greater variation in contractile velocity within the shortening phase of individual contractions. At 20-25 days (12-15\% adult chukar mass), pectoralis activity and locomotor performance approaches that of adults, although strain rate exhibits a temporary decrease at 61 dph concurrent with using newly-replaced primary feathers. To better understand how these patterns relate to the evolution of life-history strategy and locomotion, we encourage future efforts to explore these behaviors in altricial and semi-altricial bird species.",
    url = "https://pubmed.ncbi.nlm.nih.gov/28662566/",
    doi = "10.1093/icb/icx050",
    openalex = "W2732594263",
    pmid = "28662566",
    references = "doi101038164820b0, doi101038nature01284, doi101073pnas882210357, doi101086498196, doi101086physzool67430163866, doi101111j155856461996tb04496x, doi101126science1078237, doi101126scienceaad1173, doi101242jeb204152717, doi1023071445582"
}

Harnessing flight strategies refined by millions of years of evolution can help expedite the design of more efficient, manoeuvrable and robust flying robots. This review synthesizes recent advances and highlights remaining gaps in our understanding of how bird and bat wing adaptations enable effective flight. Included in this discussion is an evaluation of how current robotic analogues measure up to their biological sources of inspiration. Studies of vertebrate wings have revealed skeletal systems well suited for enduring the loads required during flight, but the mechanisms that drive coordinated motions between bones and connected integuments remain ill-described. Similarly, vertebrate flight muscles have adapted to sustain increased wing loading, but a lack of in vivo studies limits our understanding of specific muscular functions. Forelimb adaptations diverge at the integument level, but both bird feathers and bat membranes yield aerodynamic surfaces with a level of robustness unparalleled by engineered wings. These morphological adaptations enable a diverse range of kinematics tuned for different flight speeds and manoeuvres. By integrating vertebrate flight specializations-particularly those that enable greater robustness and adaptability-into the design and control of robotic wings, engineers can begin narrowing the wide margin that currently exists between flying robots and vertebrates. In turn, these robotic wings can help biologists create experiments that would be impossible in vivo.

@article{doi101098rsif20170240,
    author = "Chin, Diana D and Matloff, Laura Y and Stowers, Amanda Kay and Tucci, Emily R and Lentink, David",
    title = "Inspiration for wing design: how forelimb specialization enables active flight in modern vertebrates.",
    year = "2017",
    journal = "Journal of the Royal Society, Interface",
    abstract = "Harnessing flight strategies refined by millions of years of evolution can help expedite the design of more efficient, manoeuvrable and robust flying robots. This review synthesizes recent advances and highlights remaining gaps in our understanding of how bird and bat wing adaptations enable effective flight. Included in this discussion is an evaluation of how current robotic analogues measure up to their biological sources of inspiration. Studies of vertebrate wings have revealed skeletal systems well suited for enduring the loads required during flight, but the mechanisms that drive coordinated motions between bones and connected integuments remain ill-described. Similarly, vertebrate flight muscles have adapted to sustain increased wing loading, but a lack of in vivo studies limits our understanding of specific muscular functions. Forelimb adaptations diverge at the integument level, but both bird feathers and bat membranes yield aerodynamic surfaces with a level of robustness unparalleled by engineered wings. These morphological adaptations enable a diverse range of kinematics tuned for different flight speeds and manoeuvres. By integrating vertebrate flight specializations-particularly those that enable greater robustness and adaptability-into the design and control of robotic wings, engineers can begin narrowing the wide margin that currently exists between flying robots and vertebrates. In turn, these robotic wings can help biologists create experiments that would be impossible in vivo.",
    url = "https://pmc.ncbi.nlm.nih.gov/articles/PMC5493806/",
    doi = "10.1098/rsif.2017.0240",
    openalex = "W2622111754",
    pmcid = "PMC5493806",
    pmid = "28592663",
    references = "doi1010079783642838484, doi1010160021929088900061, doi1010160268003395000682, doi101038nature02000, doi101098rsif20150357, doi101098rspb19380050, doi101109tro2008916997, doi101126science2885463100, doi101126scienceaaa2397, doi101139cjz20150103, doi101159000109964, doi101242jeb059865, doi101371journalpbio1002074, openalexw2103608534"
}

Flapping flight is the most power-demanding mode of locomotion, associated with a suite of anatomical specializations in extant adult birds. In contrast, many developing birds use their forelimbs to negotiate environments long before acquiring "flight adaptations," recruiting their developing wings to continuously enhance leg performance and, in some cases, fly. How does anatomical development influence these locomotor behaviors? Isolating morphological contributions to wing performance is extremely challenging using purely empirical approaches. However, musculoskeletal modeling and simulation techniques can incorporate empirical data to explicitly examine the functional consequences of changing morphology by manipulating anatomical parameters individually and estimating their effects on locomotion. To assess how ontogenetic changes in anatomy affect locomotor capacity, we combined existing empirical data on muscle morphology, skeletal kinematics, and aerodynamic force production with advanced biomechanical modeling and simulation techniques to analyze the ontogeny of pectoral limb function in a precocial ground bird (Alectoris chukar). Simulations of wing-assisted incline running (WAIR) using these newly developed musculoskeletal models collectively suggest that immature birds have excess muscle capacity and are limited more by feather morphology, possibly because feathers grow more quickly and have a different style of growth than bones and muscles. These results provide critical information about the ontogeny and evolution of avian locomotion by (i) establishing how muscular and aerodynamic forces interface with the skeletal system to generate movement in morphing juvenile birds, and (ii) providing a benchmark to inform biomechanical modeling and simulation of other locomotor behaviors, both across extant species and among extinct theropod dinosaurs.

@article{doi103389fbioe201800140,
    author = "Heers, Ashley M. and Rankin, Jeffery W. and Hutchinson, John R.",
    title = "Building a Bird: Musculoskeletal Modeling and Simulation of Wing-Assisted Incline Running During Avian Ontogeny",
    year = "2018",
    journal = "Frontiers in Bioengineering and Biotechnology",
    abstract = {Flapping flight is the most power-demanding mode of locomotion, associated with a suite of anatomical specializations in extant adult birds. In contrast, many developing birds use their forelimbs to negotiate environments long before acquiring "flight adaptations," recruiting their developing wings to continuously enhance leg performance and, in some cases, fly. How does anatomical development influence these locomotor behaviors? Isolating morphological contributions to wing performance is extremely challenging using purely empirical approaches. However, musculoskeletal modeling and simulation techniques can incorporate empirical data to explicitly examine the functional consequences of changing morphology by manipulating anatomical parameters individually and estimating their effects on locomotion. To assess how ontogenetic changes in anatomy affect locomotor capacity, we combined existing empirical data on muscle morphology, skeletal kinematics, and aerodynamic force production with advanced biomechanical modeling and simulation techniques to analyze the ontogeny of pectoral limb function in a precocial ground bird (Alectoris chukar). Simulations of wing-assisted incline running (WAIR) using these newly developed musculoskeletal models collectively suggest that immature birds have excess muscle capacity and are limited more by feather morphology, possibly because feathers grow more quickly and have a different style of growth than bones and muscles. These results provide critical information about the ontogeny and evolution of avian locomotion by (i) establishing how muscular and aerodynamic forces interface with the skeletal system to generate movement in morphing juvenile birds, and (ii) providing a benchmark to inform biomechanical modeling and simulation of other locomotor behaviors, both across extant species and among extinct theropod dinosaurs.},
    url = "https://doi.org/10.3389/fbioe.2018.00140",
    doi = "10.3389/fbioe.2018.00140",
    openalex = "W2897993047",
    references = "doi1010079783319196503303511, doi101007s1027800410146, doi101007s1043900533207, doi101093icbicx050, doi101109tbme2007901024, doi10111513138397, doi10111514023390, doi10111514029304, doi101371journalpone0121476, doi105962bhltitle82303, openalexw1549114171, openalexw2593366827"
}

Electromyography (EMG) is used to understand muscle activity patterns in animals. Understanding how much variation exists in muscle activity patterns in homologous muscles across animal clades during similar behaviours is important for evaluating the evolution of muscle functions and neuromuscular control. We compared muscle activity across a range of archosaurian species and appendicular muscles, including how these EMG patterns varied across ontogeny and phylogeny, to reconstruct the evolutionary history of archosaurian muscle activation during locomotion. EMG electrodes were implanted into the muscles of turkeys, pheasants, quail, guineafowl, emus (three age classes), tinamous and juvenile Nile crocodiles across 13 different appendicular muscles. Subjects walked and ran at a range of speeds both overground and on treadmills during EMG recordings. Anatomically similar muscles such as the lateral gastrocnemius exhibited similar EMG patterns at similar relative speeds across all birds. In the crocodiles, the EMG signals closely matched previously published data for alligators. The timing of lateral gastrocnemius activation was relatively later within a stride cycle for crocodiles compared to birds. This difference may relate to the coordinated knee extension and ankle plantarflexion timing across the swing-stance transition in Crocodylia, unlike in birds where there is knee flexion and ankle dorsiflexion across swing-stance. No significant effects were found across the species for ontogeny, or between treadmill and overground locomotion. Our findings strengthen the inference that some muscle EMG patterns remained conservative throughout Archosauria: for example, digital flexors retained similar stance phase activity and M. pectoralis remained an 'anti-gravity' muscle. However, some avian hindlimb muscles evolved divergent activations in tandem with functional changes such as bipedalism and more crouched postures, especially M. iliotrochantericus caudalis switching from swing to stance phase activity and M. iliofibularis adding a novel stance phase burst of activity.

@article{doi101002jmor20973,
    author = "Cuff, Andrew R. and Daley, Monica A. and Michel, Krijn B. and Allen, Vivian and Lamas, L. P. and Adami, Chiara and Monticelli, Paolo and Pelligand, Ludovic and Hutchinson, John R.",
    title = "Relating neuromuscular control to functional anatomy of limb muscles in extant archosaurs",
    year = "2019",
    journal = "Journal of Morphology",
    abstract = "Electromyography (EMG) is used to understand muscle activity patterns in animals. Understanding how much variation exists in muscle activity patterns in homologous muscles across animal clades during similar behaviours is important for evaluating the evolution of muscle functions and neuromuscular control. We compared muscle activity across a range of archosaurian species and appendicular muscles, including how these EMG patterns varied across ontogeny and phylogeny, to reconstruct the evolutionary history of archosaurian muscle activation during locomotion. EMG electrodes were implanted into the muscles of turkeys, pheasants, quail, guineafowl, emus (three age classes), tinamous and juvenile Nile crocodiles across 13 different appendicular muscles. Subjects walked and ran at a range of speeds both overground and on treadmills during EMG recordings. Anatomically similar muscles such as the lateral gastrocnemius exhibited similar EMG patterns at similar relative speeds across all birds. In the crocodiles, the EMG signals closely matched previously published data for alligators. The timing of lateral gastrocnemius activation was relatively later within a stride cycle for crocodiles compared to birds. This difference may relate to the coordinated knee extension and ankle plantarflexion timing across the swing-stance transition in Crocodylia, unlike in birds where there is knee flexion and ankle dorsiflexion across swing-stance. No significant effects were found across the species for ontogeny, or between treadmill and overground locomotion. Our findings strengthen the inference that some muscle EMG patterns remained conservative throughout Archosauria: for example, digital flexors retained similar stance phase activity and M. pectoralis remained an 'anti-gravity' muscle. However, some avian hindlimb muscles evolved divergent activations in tandem with functional changes such as bipedalism and more crouched postures, especially M. iliotrochantericus caudalis switching from swing to stance phase activity and M. iliofibularis adding a novel stance phase burst of activity.",
    url = "https://doi.org/10.1002/jmor.20973",
    doi = "10.1002/jmor.20973",
    openalex = "W2920855211",
    references = "doi101038nature15697, doi101038nature19417, doi101093icbicx050, doi101111j109600311988tb00514x, doi101111j146979981983tb04266x, doi101111j146979981991tb04794x, doi10111514023390, doi101146annurevne08030185001313, doi101152jn19794251212, doi10230730135049, doi105860choice326223"
}

Avian wing shape is highly variable across species but only coarsely associated with flight behavior, performance, and body mass. An underexplored but potentially explanatory feature is the ability of birds to actively change wing shape to meet aerodynamic and behavioral demands. Across 61 species, we found strong associations with flight behavior and mass for range of motion traits but not wing shape and strikingly different associations for different aspects of motion capability. Further, static morphology exhibits high phylogenetic signal, whereas range of motion shows greater evolutionary lability. These results suggest a new framework for understanding the evolution of avian flight: Rather than wing morphology, it is range of motion, an emergent property of morphology, that is predominantly reshaped as flight strategy and body size evolve.

@article{doi101126sciadvaaw6670,
    author = "Baliga, Vikram B. and Szabó, Ildikò and Altshuler, Douglas L.",
    title = "Range of motion in the avian wing is strongly associated with flight behavior and body mass",
    year = "2019",
    journal = "Science Advances",
    abstract = "Avian wing shape is highly variable across species but only coarsely associated with flight behavior, performance, and body mass. An underexplored but potentially explanatory feature is the ability of birds to actively change wing shape to meet aerodynamic and behavioral demands. Across 61 species, we found strong associations with flight behavior and mass for range of motion traits but not wing shape and strikingly different associations for different aspects of motion capability. Further, static morphology exhibits high phylogenetic signal, whereas range of motion shows greater evolutionary lability. These results suggest a new framework for understanding the evolution of avian flight: Rather than wing morphology, it is range of motion, an emergent property of morphology, that is predominantly reshaped as flight strategy and body size evolve.",
    url = "https://doi.org/10.1126/sciadv.aaw6670",
    doi = "10.1126/sciadv.aaw6670",
    openalex = "W2981663850",
    references = "doi101098rsif20170240"
}

Abstract Insectivorous birds reach their highest diversity in the tropics and represent a striking variety of morphological and behavioral specializations for foraging, yet explanations for these patterns are inadequate because of both our limited understanding of the drivers of ecological diversification within and among clades and of coexistence mechanisms in particular. Here we synthesize recent information on Neotropical insectivorous birds, including their diversity, evolutionary ages and locations of origin, phylogenies, and both competitive and predator–prey species interactions. We propose a novel evolutionary hypothesis for the origin and coexistence of the phenotypic diversity of insectivore foraging morphologies in species-rich communities, based on their extraordinary food-resource specializations. Specifically, we develop the Biotic Challenge Hypothesis to explain the evolution of these specializations, and we provide preliminary evidence in support of this hypothesis based on a synopsis of both Neotropical insectivore specializations by family and arthropod antipredator adaptations by category. We argue that, from the perspective of tropical insectivorous birds, and particularly in the most species-rich, mainland Neotropical communities, the environment is an arthropod desert. Coexistence with all of the other insectivores requires feeding specialization to compete exploitatively and diffusely against evolutionarily diverse species and far less frequently against sister species. The arthropod desert arises primarily because of (1) the tactical diversity of arthropod predators as insectivore competitors and (2) the evolutionary arms races involving arthropod predators with their prey, which render many arthropods inaccessible to most insectivorous predators. Our idea provides an explicit mechanism for pervasive, diffuse tropical interspecific competition, for evolutionary specialization, and for positive feedback on speciation rates at low latitudes, thereby generating new predictions and insights into tropical life histories and the Latitudinal Diversity Gradient. Other recent ideas concerning the coexistence of Neotropical insectivores, including positive species interactions within mixed species flocks, are recognized and evaluated. We discuss ways to test predictions resulting from the new view of communities developed here, including a case study of diet specialization by Costa Rican tyrannid flycatchers. Our synthesis of the origin and nature of Neotropical insectivore communities injects new life into the “zombie” idea that evolution works differently in the species-rich tropics.

@article{doi101093aukukaa049,
    author = "Sherry, Thomas W. and Kent, Cody M. and Sánchez, Natalie V. and Şekercioḡlu, Çaḡan H.",
    title = "Insectivorous birds in the Neotropics: Ecological radiations, specialization, and coexistence in species-rich communities",
    year = "2020",
    journal = "The Auk",
    abstract = "Abstract Insectivorous birds reach their highest diversity in the tropics and represent a striking variety of morphological and behavioral specializations for foraging, yet explanations for these patterns are inadequate because of both our limited understanding of the drivers of ecological diversification within and among clades and of coexistence mechanisms in particular. Here we synthesize recent information on Neotropical insectivorous birds, including their diversity, evolutionary ages and locations of origin, phylogenies, and both competitive and predator–prey species interactions. We propose a novel evolutionary hypothesis for the origin and coexistence of the phenotypic diversity of insectivore foraging morphologies in species-rich communities, based on their extraordinary food-resource specializations. Specifically, we develop the Biotic Challenge Hypothesis to explain the evolution of these specializations, and we provide preliminary evidence in support of this hypothesis based on a synopsis of both Neotropical insectivore specializations by family and arthropod antipredator adaptations by category. We argue that, from the perspective of tropical insectivorous birds, and particularly in the most species-rich, mainland Neotropical communities, the environment is an arthropod desert. Coexistence with all of the other insectivores requires feeding specialization to compete exploitatively and diffusely against evolutionarily diverse species and far less frequently against sister species. The arthropod desert arises primarily because of (1) the tactical diversity of arthropod predators as insectivore competitors and (2) the evolutionary arms races involving arthropod predators with their prey, which render many arthropods inaccessible to most insectivorous predators. Our idea provides an explicit mechanism for pervasive, diffuse tropical interspecific competition, for evolutionary specialization, and for positive feedback on speciation rates at low latitudes, thereby generating new predictions and insights into tropical life histories and the Latitudinal Diversity Gradient. Other recent ideas concerning the coexistence of Neotropical insectivores, including positive species interactions within mixed species flocks, are recognized and evaluated. We discuss ways to test predictions resulting from the new view of communities developed here, including a case study of diet specialization by Costa Rican tyrannid flycatchers. Our synthesis of the origin and nature of Neotropical insectivore communities injects new life into the “zombie” idea that evolution works differently in the species-rich tropics.",
    url = "https://doi.org/10.1093/auk/ukaa049",
    doi = "10.1093/auk/ukaa049",
    openalex = "W3083309830",
    references = "doi101126scienceaad1173"
}

Abstract Wingtip slots, where the outer primary feathers of birds split and spread vertically, are regarded as an evolved favorable feature that could effectively improve their aerodynamic performance. They have inspired many to perform experiments and simulations as well as to relate their results to aircraft design. This paper aims to provide guidance for the research on the aerodynamic mechanism of wingtip slots. Following a review of previous wingtip slot research, four imperfections are put forward: vacancies in research content, inconsistencies in research conclusions, limitations of early research methods, and shortage of the aerodynamic mechanism analysis. On this basis, further explorations and expansion of the influence factors for steady state are needed; more attention should be poured into the application of flow field integration method to decompose drag, and evaluation of variation in induced drag seems a more rational choice. Geometric and kinematic parameters of wingtip slot structure in the unsteady state, as well as the flexibility of wingtips, should be taken into account. As for the aerodynamic mechanism of wingtip slots, the emphasis can be placed on the study of the formation, development, and evolution of wingtip vortices on slotted wings. Besides, some research strategies and feasibility analyses are proposed for each part of the research.

@article{doi101007s42235021001166,
    author = "Liu, Dan and Song, Bifeng and Yang, Wenqing and Yang, Xiaojun and Xue, Dong and Lang, Xinyu",
    title = "A Brief Review on Aerodynamic Performance of Wingtip Slots and Research Prospect",
    year = "2021",
    journal = "Journal of Bionic Engineering",
    abstract = "Abstract Wingtip slots, where the outer primary feathers of birds split and spread vertically, are regarded as an evolved favorable feature that could effectively improve their aerodynamic performance. They have inspired many to perform experiments and simulations as well as to relate their results to aircraft design. This paper aims to provide guidance for the research on the aerodynamic mechanism of wingtip slots. Following a review of previous wingtip slot research, four imperfections are put forward: vacancies in research content, inconsistencies in research conclusions, limitations of early research methods, and shortage of the aerodynamic mechanism analysis. On this basis, further explorations and expansion of the influence factors for steady state are needed; more attention should be poured into the application of flow field integration method to decompose drag, and evaluation of variation in induced drag seems a more rational choice. Geometric and kinematic parameters of wingtip slot structure in the unsteady state, as well as the flexibility of wingtips, should be taken into account. As for the aerodynamic mechanism of wingtip slots, the emphasis can be placed on the study of the formation, development, and evolution of wingtip vortices on slotted wings. Besides, some research strategies and feasibility analyses are proposed for each part of the research.",
    url = "https://doi.org/10.1007/s42235-021-00116-6",
    doi = "10.1007/s42235-021-00116-6",
    openalex = "W4200360547",
    references = "doi101093biolinneanblx130"
}

Musculoskeletal computer models allow us to quantitatively relate morphological features to biomechanical performance. In non-human apes, certain morphological features have long been linked to greater arm abduction potential and increased arm-raising performance, compared to humans. Here, we present the first musculoskeletal model of a western lowland gorilla shoulder to test some of these long-standing proposals. Estimates of moment arms and moments of the glenohumeral abductors (deltoid, supraspinatus and infraspinatus muscles) over arm abduction were conducted for the gorilla model and a previously published human shoulder model. Contrary to previous assumptions, we found that overall glenohumeral abduction potential is similar between Gorilla and Homo. However, gorillas differ by maintaining high abduction moment capacity with the arm raised above horizontal. This difference is linked to a disparity in soft tissue properties, indicating that scapular morphological features like a cranially oriented scapular spine and glenoid do not enhance the abductor function of the gorilla glenohumeral muscles. A functional enhancement due to differences in skeletal morphology was only demonstrated in the gorilla supraspinatus muscle. Contrary to earlier ideas linking a more obliquely oriented scapular spine to greater supraspinatus leverage, our results suggest that increased lateral projection of the greater tubercle of the humerus accounts for the greater biomechanical performance in Gorilla. This study enhances our understanding of the evolution of gorilla locomotion, as well as providing greater insight into the general interaction between anatomy, function and locomotor biomechanics.

@article{doi101111joa13412,
    author = "van Beesel, Julia and Hutchinson, John R. and Hublin, Jean‐Jacques and Melillo, Stephanie M.",
    title = "Exploring the functional morphology of the Gorilla shoulder through musculoskeletal modelling",
    year = "2021",
    journal = "Journal of Anatomy",
    abstract = "Musculoskeletal computer models allow us to quantitatively relate morphological features to biomechanical performance. In non-human apes, certain morphological features have long been linked to greater arm abduction potential and increased arm-raising performance, compared to humans. Here, we present the first musculoskeletal model of a western lowland gorilla shoulder to test some of these long-standing proposals. Estimates of moment arms and moments of the glenohumeral abductors (deltoid, supraspinatus and infraspinatus muscles) over arm abduction were conducted for the gorilla model and a previously published human shoulder model. Contrary to previous assumptions, we found that overall glenohumeral abduction potential is similar between Gorilla and Homo. However, gorillas differ by maintaining high abduction moment capacity with the arm raised above horizontal. This difference is linked to a disparity in soft tissue properties, indicating that scapular morphological features like a cranially oriented scapular spine and glenoid do not enhance the abductor function of the gorilla glenohumeral muscles. A functional enhancement due to differences in skeletal morphology was only demonstrated in the gorilla supraspinatus muscle. Contrary to earlier ideas linking a more obliquely oriented scapular spine to greater supraspinatus leverage, our results suggest that increased lateral projection of the greater tubercle of the humerus accounts for the greater biomechanical performance in Gorilla. This study enhances our understanding of the evolution of gorilla locomotion, as well as providing greater insight into the general interaction between anatomy, function and locomotor biomechanics.",
    url = "https://doi.org/10.1111/joa.13412",
    doi = "10.1111/joa.13412",
    openalex = "W3133111837",
    references = "doi103389fbioe201800140"
}

Bipedal locomotion evolved along the archosaurian lineage to birds, shifting from "hip-based" to "knee-based" mechanisms. However, the roles of individual muscles in these changes and their evolutionary timings remain obscure. Using 13 three-dimensional musculoskeletal models of the hindlimbs of bird-line archosaurs, we quantify how the moment arms (i.e., leverages) of 35 locomotor muscles evolved. Our results support two hypotheses: From early theropod dinosaurs to birds, knee flexors' moment arms decreased relative to knee extensors', and medial long-axis rotator moment arms for the hip increased (trading off with decreased hip abductor moment arms). Our results reveal how, from the Triassic Period, bipedal theropod dinosaurs gradually modified their hindlimb form and function, shifting more from hip-based to knee-based locomotion and hip-abductor to hip-rotator balancing mechanisms inherited by birds. Yet, we also discover unexpected ancestral specializations in larger Jurassic theropods, lost later in the bird-line, complicating the paradigm of gradual transformation.

@article{doi101126sciadvabe2778,
    author = "Allen, Vivian and Kilbourne, Brandon M. and Hutchinson, John R.",
    title = "The evolution of pelvic limb muscle moment arms in bird-line archosaurs",
    year = "2021",
    journal = "Science Advances",
    abstract = {Bipedal locomotion evolved along the archosaurian lineage to birds, shifting from "hip-based" to "knee-based" mechanisms. However, the roles of individual muscles in these changes and their evolutionary timings remain obscure. Using 13 three-dimensional musculoskeletal models of the hindlimbs of bird-line archosaurs, we quantify how the moment arms (i.e., leverages) of 35 locomotor muscles evolved. Our results support two hypotheses: From early theropod dinosaurs to birds, knee flexors' moment arms decreased relative to knee extensors', and medial long-axis rotator moment arms for the hip increased (trading off with decreased hip abductor moment arms). Our results reveal how, from the Triassic Period, bipedal theropod dinosaurs gradually modified their hindlimb form and function, shifting more from hip-based to knee-based locomotion and hip-abductor to hip-rotator balancing mechanisms inherited by birds. Yet, we also discover unexpected ancestral specializations in larger Jurassic theropods, lost later in the bird-line, complicating the paradigm of gradual transformation.},
    url = "https://doi.org/10.1126/sciadv.abe2778",
    doi = "10.1126/sciadv.abe2778",
    openalex = "W3137235955",
    references = "doi101002jmor20973, doi1010800272463420171427593"
}

The evolution of avian flight is one of the great transformations in vertebrate history, marked by striking anatomical changes that presumably help meet the demands of aerial locomotion. These changes did not occur simultaneously, and are challenging to decipher. Although extinct theropods are most often compared to adult birds, studies show that developing birds can uniquely address certain challenges and provide powerful insights into the evolution of avian flight: unlike adults, immature birds have rudimentary, somewhat “dinosaur-like” flight apparatuses and can reveal relationships between form, function, performance, and behavior during flightless to flight-capable transitions. Here, we focus on the musculoskeletal apparatus and use CT scans coupled with a three-dimensional musculoskeletal modeling approach to analyze how ontogenetic changes in skeletal anatomy influence muscle size, leverage, orientation, and corresponding function during the development of flight in a precocial ground bird (Alectoris chukar). Our results demonstrate that immature and adult birds use different functional solutions to execute similar locomotor behaviors: in spite of dramatic changes in skeletal morphology, muscle paths and subsequent functions are largely maintained through ontogeny, because shifts in one bone are offset by changes in others. These findings help provide a viable mechanism for how extinct winged theropods with rudimentary pectoral skeletons might have achieved bird-like behaviors before acquiring fully bird-like anatomies. These findings also emphasize the importance of a holistic, whole-body perspective, and the need for extant validation of extinct behaviors and performance. As empirical studies on locomotor ontogeny accumulate, it is becoming apparent that traditional, isolated interpretations of skeletal anatomy mask the reality that integrated whole systems function in frequently unexpected yet effective ways. Collaborative and integrative efforts that address this challenge will surely strengthen our exploration of life and its evolutionary history.

@article{doi103389fevo2020573411,
    author = "Heers, Ashley M. and Varghese, S. and Hatier, Leila K. and Cabrera, Jeremiah J.",
    title = "Multiple Functional Solutions During Flightless to Flight-Capable Transitions",
    year = "2021",
    journal = "Frontiers in Ecology and Evolution",
    abstract = "The evolution of avian flight is one of the great transformations in vertebrate history, marked by striking anatomical changes that presumably help meet the demands of aerial locomotion. These changes did not occur simultaneously, and are challenging to decipher. Although extinct theropods are most often compared to adult birds, studies show that developing birds can uniquely address certain challenges and provide powerful insights into the evolution of avian flight: unlike adults, immature birds have rudimentary, somewhat “dinosaur-like” flight apparatuses and can reveal relationships between form, function, performance, and behavior during flightless to flight-capable transitions. Here, we focus on the musculoskeletal apparatus and use CT scans coupled with a three-dimensional musculoskeletal modeling approach to analyze how ontogenetic changes in skeletal anatomy influence muscle size, leverage, orientation, and corresponding function during the development of flight in a precocial ground bird (Alectoris chukar). Our results demonstrate that immature and adult birds use different functional solutions to execute similar locomotor behaviors: in spite of dramatic changes in skeletal morphology, muscle paths and subsequent functions are largely maintained through ontogeny, because shifts in one bone are offset by changes in others. These findings help provide a viable mechanism for how extinct winged theropods with rudimentary pectoral skeletons might have achieved bird-like behaviors before acquiring fully bird-like anatomies. These findings also emphasize the importance of a holistic, whole-body perspective, and the need for extant validation of extinct behaviors and performance. As empirical studies on locomotor ontogeny accumulate, it is becoming apparent that traditional, isolated interpretations of skeletal anatomy mask the reality that integrated whole systems function in frequently unexpected yet effective ways. Collaborative and integrative efforts that address this challenge will surely strengthen our exploration of life and its evolutionary history.",
    url = "https://doi.org/10.3389/fevo.2020.573411",
    doi = "10.3389/fevo.2020.573411",
    openalex = "W3128315123",
    references = "doi103389fbioe201800140"
}

Flight speed is positively correlated with body size in animals 1. However, miniature featherwing beetles can fly at speeds and accelerations of insects three times their size 2. Here we show that this performance results from a reduced wing mass and a previously unknown type of wing-motion cycle. Our experiment combines three-dimensional reconstructions of morphology and kinematics in one of the smallest insects, the beetle Paratuposa placentis (body length 395 μm). The flapping bristled wings follow a pronounced figure-of-eight loop that consists of subperpendicular up and down strokes followed by claps at stroke reversals above and below the body. The elytra act as inertial brakes that prevent excessive body oscillation. Computational analyses suggest functional decomposition of the wingbeat cycle into two power half strokes, which produce a large upward force, and two down-dragging recovery half strokes. In contrast to heavier membranous wings, the motion of bristled wings of the same size requires little inertial power. Muscle mechanical power requirements thus remain positive throughout the wingbeat cycle, making elastic energy storage obsolete. These adaptations help to explain how extremely small insects have preserved good aerial performance during miniaturization, one of the factors of their evolutionary success.

@article{doi101038s41586021043037,
    author = "Farisenkov, S. E. and Kolomenskiy, Dmitry and Petrov, Pyotr N. and Engels, Thomas and Lapina, Nadezhda A. and Lehmann, Fritz‐Olaf and Onishi, Ryo and Liu, Hao and Polilov, Alexey A.",
    title = "Novel flight style and light wings boost flight performance of tiny beetles",
    year = "2022",
    journal = "Nature",
    abstract = "Flight speed is positively correlated with body size in animals 1. However, miniature featherwing beetles can fly at speeds and accelerations of insects three times their size 2. Here we show that this performance results from a reduced wing mass and a previously unknown type of wing-motion cycle. Our experiment combines three-dimensional reconstructions of morphology and kinematics in one of the smallest insects, the beetle Paratuposa placentis (body length 395 μm). The flapping bristled wings follow a pronounced figure-of-eight loop that consists of subperpendicular up and down strokes followed by claps at stroke reversals above and below the body. The elytra act as inertial brakes that prevent excessive body oscillation. Computational analyses suggest functional decomposition of the wingbeat cycle into two power half strokes, which produce a large upward force, and two down-dragging recovery half strokes. In contrast to heavier membranous wings, the motion of bristled wings of the same size requires little inertial power. Muscle mechanical power requirements thus remain positive throughout the wingbeat cycle, making elastic energy storage obsolete. These adaptations help to explain how extremely small insects have preserved good aerial performance during miniaturization, one of the factors of their evolutionary success.",
    url = "https://doi.org/10.1038/s41586-021-04303-7",
    doi = "10.1038/s41586-021-04303-7",
    openalex = "W4205881884",
    references = "doi101098rstb20150388"
}

Birds generate a propulsive force by flapping their wings. They use this propulsive force for various locomotion styles, such as aerodynamic flight, wing-paddle swimming and wing-assisted incline running. It is therefore important to reveal the origin of flapping ability in the evolution from theropod dinosaurs to birds. However, there are no quantitative indices to reconstruct the flapping abilities of extinct forms based on their skeletal morphology. This study compares the section modulus of the coracoid relative to body mass among various extant birds to test whether the index is correlated with flapping ability. According to a survey of 220 historical bird specimens representing 209 species, 180 genera, 83 families and 30 orders, the section modulus of the coracoid relative to body mass in non-flapping birds was significantly smaller than that of flapping birds. This indicates that coracoid strength in non-flapping birds is deemphasised, whereas in flapping birds the strength is emphasised to withstand the contractile force produced by powerful flapping muscles, such as the m. pectoralis and m. supracoracoideus. Therefore, the section modulus of the coracoid is expected to be a powerful tool to reveal the origin of powered flight in birds.

@article{doi101111joa13788,
    author = "Akeda, Takumi and Fujiwara, Shin‐ichi",
    title = "Coracoid strength as an indicator of wing‐beat propulsion in birds",
    year = "2022",
    journal = "Journal of Anatomy",
    abstract = "Birds generate a propulsive force by flapping their wings. They use this propulsive force for various locomotion styles, such as aerodynamic flight, wing-paddle swimming and wing-assisted incline running. It is therefore important to reveal the origin of flapping ability in the evolution from theropod dinosaurs to birds. However, there are no quantitative indices to reconstruct the flapping abilities of extinct forms based on their skeletal morphology. This study compares the section modulus of the coracoid relative to body mass among various extant birds to test whether the index is correlated with flapping ability. According to a survey of 220 historical bird specimens representing 209 species, 180 genera, 83 families and 30 orders, the section modulus of the coracoid relative to body mass in non-flapping birds was significantly smaller than that of flapping birds. This indicates that coracoid strength in non-flapping birds is deemphasised, whereas in flapping birds the strength is emphasised to withstand the contractile force produced by powerful flapping muscles, such as the m. pectoralis and m. supracoracoideus. Therefore, the section modulus of the coracoid is expected to be a powerful tool to reveal the origin of powered flight in birds.",
    url = "https://doi.org/10.1111/joa.13788",
    doi = "10.1111/joa.13788",
    openalex = "W4309163818",
    references = "doi101017s1751731117001896"
}

Dielectric elastomers (DE), renowned for their lightweight, rapid response, high energy density, and efficient conversion, have garnered significant attention in the realm of avian flight bionics. However, a lack of understanding of the mechanical principles underlying flapping wing biomimetics has hindered accurate simulations of actual bird flight postures. To address this, a stiffness-variable DE-based wing (DEW) has been designed, inspired by the characteristics of seagulls, to mimic the wing deformations across different bird flight phases. The evolution of the DEW’s deformation is modeled based on the differential equation describing the bending curve of a DE cantilever beam. By applying different voltage cycles, three continuous avian flight postures have been replicated: takeoff, cruising, and hovering. The simulation results demonstrate that the stiffness-variable DEW effectively mimics the wing deformations observed in birds during different flight postures, closely resembling real-world flight conditions. This study has the potential to serve as an important reference for exploring changes in bird flight behavior, thereby advancing the application of DE materials in the field of soft robotics.

@article{doi1010801947541120242414292,
    author = "Ci, Haihao and Guo, Zhansheng",
    title = "Design of stiffness-variable dielectric elastomer wing based on seagull characteristics and its application in avian flight bionics",
    year = "2024",
    journal = "International Journal of Smart and Nano Materials",
    abstract = "Dielectric elastomers (DE), renowned for their lightweight, rapid response, high energy density, and efficient conversion, have garnered significant attention in the realm of avian flight bionics. However, a lack of understanding of the mechanical principles underlying flapping wing biomimetics has hindered accurate simulations of actual bird flight postures. To address this, a stiffness-variable DE-based wing (DEW) has been designed, inspired by the characteristics of seagulls, to mimic the wing deformations across different bird flight phases. The evolution of the DEW’s deformation is modeled based on the differential equation describing the bending curve of a DE cantilever beam. By applying different voltage cycles, three continuous avian flight postures have been replicated: takeoff, cruising, and hovering. The simulation results demonstrate that the stiffness-variable DEW effectively mimics the wing deformations observed in birds during different flight postures, closely resembling real-world flight conditions. This study has the potential to serve as an important reference for exploring changes in bird flight behavior, thereby advancing the application of DE materials in the field of soft robotics.",
    url = "https://doi.org/10.1080/19475411.2024.2414292",
    doi = "10.1080/19475411.2024.2414292",
    openalex = "W4403781343",
    references = "doi101016jcja202206009"
}