The claw geometry of birds can be used to predict their mode of life. Previous studies, however, have not considered how bird size might affect these predictions. Thus, in the present study, the geometric scaling relationships of bird claws are examined for a variety of extant birds with different modes of life: predatory, climbing, perching or ground-dwelling. Measurements of hind-limb claw radius (i.e. claw size) and claw angle (i.e. claw ‘hookedness’) of the third digit claw were made on 120 species of bird ranging from 0.0057 kg to 44 kg in body mass. Claw radius was found to be proportional to (body mass)0.34 across all species. Claw angle was found to increase with body mass for predatory and climbing birds (i.e. bigger birds have relatively more hooked claws), and decrease with body mass for ground-dwelling birds (i.e. bigger birds have relatively less hooked or flatter claws). No significant relationship was found between claw angle and body mass for perching birds. Mode of life could not be predicted with any certainty using measurements of either claw radius or claw angle, suggesting difficulty in assigning fossil species such as Archaeopteryx to a specific locomotor category. As claw design should enable the claw to withstand the forces placed upon it, further work is needed to establish the stresses experienced by the claws of different types of bird.

Flight—defined as the ability to produce useful aerodynamic forces by flapping wings—is one of the most demanding adaptations in vertebrates. The mechanical problems of flight ensure considerable external morphological homogeneity and behavioural similarity in extant fliers. Observations of the vortex wakes and wingbeat geometry of modern birds and bats confirm that the two groups are mechanically very similar, despite differences in phylogeny, anatomy and physiology. With this background it is possible to attack two problems: the evolution of flight in vertebrates, and the flight performance of extinct animals such as pterosaurs and Archaeopteryx.

The origin of flight has been surrounded by considerable controversy, due in part to terminological inconsistencies, in part to phylogenetic uncertainty over the relationships of birds, bats and pterosaurs, in part to disagreement over the interpretation of the available fossil evidence, and in part to argument over the relative importance of morphological, mechanical and ecological specialisations. The mechanical changes needed in the course of the evolution of flight favour a gliding origin of tetrapod flight, and on mechanical and ecological grounds the alternative cursorial hypothesis may be discounted. This argument is particularly strong in bats, but has been thought to be weaker in birds owing to apparent inconsistencies with the fossil evidence. However, fossils of the Jurassic theropod dinosaur Archaeopteryx also support a gliding origin for flight, and suggest that this animal was adapted for flapping flight at moderately high speeds associated with gliding; it could fly less well at the slow speeds which would have been required for incipient flight in a running cursor, and at which the wingbeat is aerodynamically and kinematically considerably more complex. Slow flight in birds and bats is the more derived condition, and vertebrate flapping flight apparently evolved through a gliding stage.

The pterosaurs have become the subject of much controversy over the nature of their stance, the wing surface, and the degree of involvement of the leg in the wing membrane. Reconstruction of their wings indicates proficient flying animals, and comparison with birds suggests that most pterosaurs probably occupied marine or coastal/estuarine habitat.