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In dictating the relative distances between the elbow, shoulder and wrist, avian brachial index (BI = humerus/radius-ulna length) likely influences wing kinematics and, therefore, might predict extinct avian flight capability. This underpins the hypothesis that non-neornithine Mesozoic avialans with relatively low BIs (associated with improved flight capabilities) restricted neornithine diversification until after the Cretaceous-Paleogene boundary. Here, correlations between flight metrics (wingbeat frequency (f), stroke angle (θ), wing loading (Q) and aspect ratio) and BI were investigated and vice versa. Additionally, the evolutionary model best describing the phylogenetic distribution of BI, and the temporal patterns in BI, flight metrics, body mass (M b ), and size-corrected humerus (L h ) and radius-ulna (L ru ) length were determined. BI was best described by Ornstein-Uhlenbeck processes, with low α values indicating a gradual shift towards a future theoretical optimum. BI also decreased overall through evolutionary time with the flight metrics mirroring temporal patterns of change in BI. M b , L h and L ru overall decreased apart from increases in L h and L ru following the middle-late Miocene (also leading to BI increasing) due to diversifications of the Anatinae and Sphenisciformes. L h overall decreased further than L ru. Consequently, decreasing L h mainly contributed to decreasing BI through evolutionary time, implying flight performance increased through neornithine evolution. However, the timings of radiations in these variables implies an Eocene radiation of neornthine flight ecology rather than a rapid expansion into niches vacated by non-neornithine Mesozoic avialians following the Cretaceous-Paleogene boundary. Multiple regressions showed f, θ and Q explained 60% of variation in BI. However, unequivocally evaluating whether BI is related to wing movement (and flight capability) requires direct measures of wing movement for many species, which are currently unavailable. Finally, the findings here and previously observed clade-specificity in BI, suggest flight ecology may also be clade-specific. Hence, the utility of phylogeny in predicting flight ecology requires exploration., (© 2021 Wiley Periodicals LLC.)

Extinct animal behavior has often been inferred from qualitative assessments of relative brain region size in fossil endocranial casts. For instance, flight capability in pterosaurs and early birds has been inferred from the relative size of the cerebellar flocculus, which in life protrudes from the lateral surface of the cerebellum. A primary role of the flocculus is to integrate sensory information about head rotation and translation to stabilize visual gaze via the vestibulo-occular reflex (VOR). Because gaze stabilization is a critical aspect of flight, some authors have suggested that the flocculus is enlarged in flying species. Whether this can be further extended to a floccular expansion in highly maneuverable flying species or floccular reduction in flightless species is unknown. Here, we used micro computed-tomography to reconstruct "virtual" endocranial casts of 60 extant bird species, to extract the same level of anatomical information offered by fossils. Volumes of the floccular fossa and entire brain cavity were measured and these values correlated with four indices of flying behavior. Although a weak positive relationship was found between floccular fossa size and brachial index, no significant relationship was found between floccular fossa size and any other flight mode classification. These findings could be the result of the bony endocranium inaccurately reflecting the size of the neural flocculus, but might also reflect the importance of the flocculus for all modes of locomotion in birds. We therefore conclude that the relative size of the flocculus of endocranial casts is an unreliable predictor of locomotor behavior in extinct birds, and probably also pterosaurs and non-avian dinosaurs.

The relative length proportions of the three bony elements of the pelvic (femur, tibiotarsus and tarsometatarsus) and pectoral (humerus, ulna and manus) limbs of the early Cretaceous bird Gansus yumenensis, a well-represented basal ornithuromorph from China, are investigated and compared to those of extant taxa. Ternary plots show that the pectoral limb length proportions of Gansus are most similar to Apodiformes (swifts and hummingbirds), which plot away from all other extant birds. In contrast, the pelvic limb length proportions of Gansus fall within the extant bird cluster and show similarities with the neornithine families Podicipedidae (grebes), Diomedeidae (albatross) and Phalacrocoracidae (cormorants). Although it does have some of the pelvic limb features of grebes and cormorants, the femur of Gansus is more gracile and is thus more consistent with an albatross-like shallow-diving mode of life than a strong foot-propelled diving movement pattern. The position of Gansus in pectoral limb ternary morphospace is largely due to its elongated manus. In contrast to apodiformes, where the humerus and ulna are short and robust, an adaptation, which provides a stiff wing for their demanding fast agile and hovering flight (respectively), the wing-bones of Gansus are slender, indicating a less vigorous flapping flight style. The suite of characters exhibited by Gansus mean it is difficult to completely interpret its likely ecology. Nevertheless, our analyses suggest that it is probable that this bird was both volant and capable of diving to some degree using either foot-propelled or, perhaps, both its wings and its feet for underwater locomotion., (© 2012 The Authors. Journal of Evolutionary Biology © 2012 European Society For Evolutionary Biology.)

The primary feathers of birds are subject to cyclical forces in flight causing their shafts (rachises) to bend. The amount the feathers deflect during flight is dependent upon the flexural stiffness of the rachises. By quantifying scaling relationships between body mass and feather linear dimensions in a large data set of living birds, we show that both feather length and feather diameter scale much closer to predictions for geometric similarity than they do to elastic similarity. Scaling allometry also indicates that the primary feathers of larger birds are relatively shorter and their rachises relatively narrower, compared to those of smaller birds. Two-point bending tests indicated that larger birds have more flexible feathers than smaller species. Discriminant functional analyses (DFA) showed that body mass, primary feather length and rachis diameter can be used to differentiate between different magnitudes of feather bending stiffness, with primary feather length explaining 63% of variance in rachis stiffness. Adding fossil measurement data to our DFA showed that Archaeopteryx and Confuciusornis do not overlap with extant birds. This strongly suggests that the bending stiffness of their primary feathers was different to extant birds and provides further evidence for distinctive flight styles and likely limited flight ability in Archaeopteryx and Confuciusornis., (© 2012 The Authors. Journal of Evolutionary Biology © 2012 European Society For Evolutionary Biology.)

We examine the relationships between primary feather length (f(prim)) and total arm length (ta) (sum of humerus, ulna and manus lengths) in Mesozoic fossil birds to address one aspect of avian wing shape evolution. Analyses show that there are significant differences in the composition of the wing between the known lineages of basal birds and that mean f(prim) (relative to ta length) is significantly shorter in Archaeopteryx and enantiornithines than it is in Confuciusornithidae and in living birds. Based on outgroup comparisons with nonavian theropods that preserve forelimb primary feathers, we show that the possession of a relatively shorter f(prim) (relative to ta length) must be the primitive condition for Aves. There is also a clear phylogenetic trend in relative primary feather length throughout bird evolution: our analyses demonstrate that the f(prim)/ta ratio increases among successive lineages of Mesozoic birds towards the crown of the tree ('modern birds'; Neornithes). Variance in this ratio also coincides with the enormous evolutionary radiation at the base of Neornithes. Because the f(prim)/ta ratio is linked to flight mode and performance in living birds, further comparisons of wing proportions among Mesozoic avians will prove informative and certainly imply that the aerial locomotion of the Early Cretaceous Confuciusornis was very different to other extinct and living birds., (© 2011 The Authors. Journal of Evolutionary Biology © 2011 European Society For Evolutionary Biology.)

The evolution of the avian wing has long fascinated biologists, yet almost no work includes the length of primary feathers in consideration of overall wing length variation. Here we show that the length of the longest primary feather (f(prim)) contributing to overall wing length scales with negative allometry against total arm (ta = humerus+ulna+manus). The scaling exponent varied slightly, although not significantly so, depending on whether a species level analysis was used or phylogeny was controlled for using independent contrasts: f(prim) is proportional to ta(0.78-0.82). The scaling exponent was not significantly different from that predicted (0.86) by earlier work. It appears that there is a general trend for the primary feathers of birds to contribute proportionally less, and ta proportionally more, to overall wingspan as this dimension increases. Wingspan in birds is constrained close to mass (M(1/3)) because of optimisation for lift production, which limits opportunities for exterior morphological change. Within the wing, variations in underlying bone and feather lengths nevertheless may, in altering the joint positions, permit a range of different flight styles by facilitating variation in upstroke kinematics.

The fossil birds Archaeopteryx and Confuciusornis had feathered wings resembling those of living birds, but their flight capabilities remain uncertain. Analysis of the rachises of their primary feathers shows that the rachises were much thinner and weaker than those of modern birds, and thus the birds were not capable of flight. Only if the primary feather rachises were solid in cross-section (the strongest structural configuration), and not hollow as in living birds, would flight have been possible. Hence, if Archaeopteryx and Confuciusornis were flapping flyers, they must have had a feather structure that was fundamentally different from that of living birds. Alternatively, if they were only gliders, then the flapping wing stroke must have appeared after the divergence of Confuciusornis, likely within the enantiornithine or ornithurine radiations.

Ontogenetic and behavioral studies using birds currently do not document the early evolution of flight because birds (including juveniles) used in such studies employ forelimb oscillation frequencies over 10 Hz, forelimb stroke-angles in excess of 130 degrees , and possess uniquely avian flight musculatures. Living birds are an advanced morphological stage in the development of flapping flight. To gain insight into the early stages of flight evolution (i.e., prebird), in the absence of a living analogue, a new approach using Strouhal number was used. Strouhal number is a nondimensional number that describes the relationship between wing-stroke amplitude (A), wing-beat frequency (f), and flight speed (U). Calculations indicated that even moderate wing movements are enough to generate rudimentary thrust and that a propulsive flapping flight-stroke could have evolved via gradual incremental changes in wing movement and wing morphology. More fundamental to the origin of the avian flapping flight-stroke is the question of how a symmetrical forelimb posture-required for gliding and flapping flight-evolved from an alternating forelimb motion, evident in all extant bipeds when running except birds.

By analyzing a homogenous dataset we show, in contradiction to a previous study, that the scaling of body frontal area (S(b)) with body mass (m(b)) does not differ between passerine and nonpasserine birds. It is likely that comparison of data collected from live passerines with data collected from frozen nonpasserines had led to the incorrect conclusion that the scaling of S(b) varied between the taxa. We suggest that body dimensions collected from frozen specimens, or specimens stored in alcohol, are not applicable to live birds, and that both the current equations presented in the literature for predicting S(b) from m(b) may lead to inaccurate estimates. Using data from preserved specimens, we found that S(b) scales isometrically with m(b) (S(b) proportional, variant m(b) (0.66)), and therefore we found no evidence for larger birds being more streamlined than smaller birds. S(b) scales with negative allometry against wingspan (b), however, and b scales with positive allometry against m(b), so larger birds have smaller S(b) relative to b. In addition, it appears that dorsoventral flattening of the body is a general characteristic of bird's bodies but that it is more pronounced in larger birds, suggesting perhaps a function in terms of increased lift during forward flight. It appears that bird's bodies obey the surface-to-area geometric scaling law, but bird body shape may vary in relation to aerodynamic function. We suggest that a large-scale study, simultaneously measuring S(b) and m(b) in live passerines and nonpasserines, is required to improve the predictive power of S(b) upon m(b) scaling equations, which play a key role in the estimation of mechanical power consumption in flight in birds. Furthermore, the relations between bird body shape and axial skeleton dimensions, with reference to aerodynamic adaptation, warrant further investigation.

The Mesozoic fossil record has proved critical for understanding the early evolution and subsequent radiation of birds. Little is known, however, about its relative completeness: just how 'good' is the fossil record of birds from the Mesozoic? This question has come to prominence recently in the debate over differences in estimated dates of origin of major clades of birds from molecular and palaeontological data. Using a dataset comprising all known fossil taxa, we present analyses that go some way towards answering this question. Whereas avian diversity remains poorly represented in the Mesozoic, many relatively complete bird specimens have been discovered. New taxa have been added to the phylogenetic tree of basal birds, but its overall shape remains constant, suggesting that the broad outlines of early avian evolution are consistently represented: no stage in the Mesozoic is characterized by an overabundance of scrappy fossils compared with more complete specimens. Examples of Neornithes (modern orders) are known from later stages in the Cretaceous, but their fossils are rarer and scrappier than those of basal bird groups, which we suggest is a biological, rather than a geological, signal.

The wing kinematics of birds vary systematically with body size, but we still, after several decades of research, lack a clear mechanistic understanding of the aerodynamic selection pressures that shape them. Swimming and flying animals have recently been shown to cruise at Strouhal numbers (St) corresponding to a regime of vortex growth and shedding in which the propulsive efficiency of flapping foils peaks (St approximately fA/U, where f is wingbeat frequency, U is cruising speed and A approximately bsin(theta/2) is stroke amplitude, in which b is wingspan and theta is stroke angle). We show that St is a simple and accurate predictor of wingbeat frequency in birds. The Strouhal numbers of cruising birds have converged on the lower end of the range 0.2 < St < 0.4 associated with high propulsive efficiency. Stroke angle scales as theta approximately 67b-0.24, so wingbeat frequency can be predicted as f approximately St.U/bsin(33.5b-0.24), with St0.21 and St0.25 for direct and intermittent fliers, respectively. This simple aerodynamic model predicts wingbeat frequency better than any other relationship proposed to date, explaining 90% of the observed variance in a sample of 60 bird species. Avian wing kinematics therefore appear to have been tuned by natural selection for high aerodynamic efficiency: physical and physiological constraints upon wing kinematics must be reconsidered in this light.

Analysis of a comprehensive dataset demonstrates that the brachial index (BI = humerus length/ulna length) of modern birds (Neornithes) varies significantly between clades at all taxonomic levels, yet is strongly correlated with recent phylogenetic hypotheses. Variance in BI at the infraclass level is low, but increases rapidly during the proposed major radiation of neornithines in the Palaeocene and Eocene. Although a BI of greater than 1 is primitive for Neornithes, more basal groups of Mesozoic birds (Confuciusornithidae and some members of the diverse Enantiornithidae) had BIs comparable with those of 'higher' modern clades. It is possible that occupation of ecological niches by these Mesozoic clades precluded the divergence of some groups of neornithines until after the Cretaceous-Tertiary boundary. We suggest that with further analysis and data collection the relationships between flight behaviour, ecology and BI can be determined. Hence, BI may provide a useful tool for characterizing the ecology of fossil birds.

Dimensionless numbers are important in biomechanics because their constancy can imply dynamic similarity between systems, despite possible differences in medium or scale. A dimensionless parameter that describes the tail or wing kinematics of swimming and flying animals is the Strouhal number, St = fA/U, which divides stroke frequency (f) and amplitude (A) by forward speed (U). St is known to govern a well-defined series of vortex growth and shedding regimes for airfoils undergoing pitching and heaving motions. Propulsive efficiency is high over a narrow range of St and usually peaks within the interval 0.2 < St < 0.4 (refs 3-8). Because natural selection is likely to tune animals for high propulsive efficiency, we expect it to constrain the range of St that animals use. This seems to be true for dolphins, sharks and bony fish, which swim at 0.2 < St < 0.4. Here we show that birds, bats and insects also converge on the same narrow range of St, but only when cruising. Tuning cruise kinematics to optimize St therefore seems to be a general principle of oscillatory lift-based propulsion.