Bird feet are adapted to the life they lead, and the patterns are often use in avian classification. They are derived from the basic limb of ancestral tetrapods, the pentadactyl limb. Whereas humans (and other primates) have the original five fingers and five toes, birds all have only four toes. These four toes are arranged into four main patterns.
- Anisodactyly. This is the most common arrangement of toes in birds, with three toes forward and one back. It is the basic pattern in passerine birds, especially the songbirds, and hunting birds like eagles, hawks, and falcons.
The anisodactyl arrangement enables the passerine birds to perch upon vertical surfaces, such as trees and cliffs. The hind toe joins the leg at the same level as the front toes.
The leg arrangement of passerine birds contains a special adaption for perching. A tendon in the rear of the leg running from the underside of the toes to the muscle behind the tibiotarsus will automatically be pulled and tighten when the leg bends, causing the foot to curl and become stiff when the bird lands on a branch. This enables passerines to sleep while perching without falling off. This is especially useful for passerine birds that develop nocturnal lifestyles.
Certain species of passerines have stiff tail feathers, which help the birds balance themselves when perching on vertical surfaces.
Syndactyly is like anisodactyly, except that the third and fourth toes or three toes, are fused together. This is characteristic of Coraciiformes (Kingfishers, Bee-eaters, Rollers, and relatives).
- Zygodactyly. This has two toes facing forward (digits 2 and 3) and two back (digits 1 and 4). This arrangement is most common in arboreal species, particularly those that climb tree trunks or clamber through foliage. Zygodactyly occurs in the parrots, woodpeckers (including flickers), cuckoos (including roadrunners), and some owls. Zygodactyl tracks have been found dating to 120–110 million years ago (Lower Cretaceous), 50 million years before the first identified zygodactyl fossils.
- Heterodactyly. Heterodactyly is like zygodactyly, except that digits 3 and 4 point forward and digits 1 and 2 point back. This is found only in trogons.
- Pamprodactyly is an arrangement where all four toes may point forward, or birds may rotate the outer two toes backward. It is a characteristic of swifts (Apodidae).
- Didactyly. The ostrich has just two toes on each foot, with the nail on the larger, inner toe resembling a hoof. The outer toe has no nail. The reduced number of toes is an adaptation to its life on open land. Ostriches can run at a speed over 70 km/h (43 mph) and can cover 3 to 5 m (9.8 to 16.4 ft) in a single stride.
The scales of birds are made of the same keratin as beaks, claws, and spurs. They are found on the toes and sometimes further up on the ankle. The scales and scutes of birds are thought to be homologous with those of reptiles, and feathers have developed from the scales of Archosaurs.
- Proctor N.S. & Lynch P.J. 1998. Manual of Ornithology: avian structure & function. Yale University Press. ISBN 0300076193
- Stefoff, Rebecca 2008. The bird class. Marshall Cavendish Benchmark.
- Brooke, Michael and Tim Birkhead 1991. The Cambridge Encyclopedia of Ornitholigy. Cambridge University Press.
- "Earliest zygodactyl bird feet: evidence from Early Cretaceous roadrunner-like tracks". Naturwissenschaften. 2007.
- San Diego Zoo's Animal Bytes: Ostrich. Sandiegozoo.org. Retrieved on 2012-08-21.
- Feduccia, Alan 1999. The origin and evolution of birds. 2nd ed, Yale University Press, p132/3 The first feathers as aerodynamic structures.