Though seemingly insignificant to an animal primarily on the wing, bird feet serve a surprising purpose, both in the locomotion and ecology of a species.
Structure[]
At first glance, a bird's legs may appear to bend backward, but this is only due to the visibility of the heel and ankle joint and the concealment of the knee joint that bends forward. The knee joint is the fusion point between the femur and tibiotarsus, capped with the patella, or kneecap.
The femur is the highest bone on the foot and is quite reduced and short in some birds. Some of the lower bones of the foot (the distals and most of the metatarsal) are fused to form the tarsometatarsus – a third segment of the leg specific to birds. The upper bones of the foot (proximals), in turn, are fused with the tibia to form the tibiotarsus, as over time the centralia disappeared.
Other bones follow and fuse to form the tarsometatarsus, where the toes of the leg meet. Birds have the peculiarity of walking on their toes due to the configuration of their fingers, so it can be said that they are digitigrade. The legs are attached to a strong assembly consisting of the pelvic girdle extensively fused with the uniform spinal bone (also specific to birds) called the synsacrum, built from some of the fused bones.
Most of birds have four digits, but some types of birds have three, with the first digit being the hallux. The ostrich (Struthio camelus) is the only living bird that has only two fingers. Those that have only three are generally other ratites such as the rhea, the emu, the kiwis, and some shorebirds such as plovers (order Charadriiformes), among others.
Types of bird feet[]
Anisodactyl[]
Anisodactyly feet are displayed in hawks, such as this Red-tailed Hawk.
The most common arrangement of digits, this type of bird feet has four toes in which the hallux (first finger) faces backwards and the other three point forward. This arrangement defines passerines (Passeriformes) and can also be found in pigeons (Columbiformes), hawks (Accipitriformes), and among many other birds. Birds with anisodactyly feet have a strong hallux that allows them to perch comfortably.
Zygodactyl[]
The parrot's zygodactyl feet are helpful in grasping food.
Birds with zygodactyl feet have two digits pointing forward and two digits pointing back. Generally, the fourth digit and hallux are the toes pointing back. This type of bird feet is found in cuckoos (Cuculiformes), woodpeckers (Piciformes) and parrots (Psittaciformes), among others. Owls (Strigiformes) also exhibit this arrangement, but it varies within the order. Having two digits in the front and back is certainly advantageous in terms of grasping. Parrots use their feet to grasp food, woodpeckers to cling to tree bark, and owls to better hold prey. Unlike all other zygodactyl birds, owls are the only order that can rotate their fourth toe forward.
Heterodactyl[]
Specific to the trogon, this rare arrangement involves the hallux and second digit turned back and the third and fourth turned forward. Like the anisodactyly arrangement, this organization is also adapted for perching.
Syndactyl[]
The third and fourth toes of the Common Kingfisher are fused together.
The third and fourth toes are connected for most of their length and have a broad sole in common. It is typical of kingfishers, bee-eaters, northern caracara, and their allies (Coraciiformes). There can also be a fusion of the front three fingers, the second to fourth, as seen in the Giant Kingfisher. This arrangement of digits allows birds to perch on flat as well as cylindrical surfaces.
Pamprodactyl[]
Found only in swifts (Apodiformes), this type of foot arrangement features all four digits pointing forward. This foot type allows swifts to hang from branches or other structures.
Tridactyl[]
In the case of the Northern Three-toed Woodpecker, the hallux is missing.
Didactyl[]
This arrangement only has two toes. It is only found in the ostrich.
Webbed Feet[]
Palmate[]
The most common arrangement of webbed feet in birds. The hallux points backward while the 2nd, 3rd, and 4th toe point forward and are connected by webbing. Birds that display this foot type are typically of the Anseriformes family.
Semipalmate[]
Similar to palmate, but webbing is reduced. Can be found in sandpipers, plovers, herons, grouse, and avocets.
Lobate[]
The hallux points backward in this arrangement and the 2nd, 3rd, and 4th toe point forward. The forward-pointing toes have lobes of skin surrounding them. Birds that have lobate feet include coots, grebes, and phalaropes.
Totipalmate[]
All four digits are connected by ample webbing. Cormorants, pelicans, anhingas, boobies, frigatebirds, and gannets have this type of feet.