Pieds d’Oiseaux - The Feet of Birds
You can clearly see the wide range of foot structures found in the Neornithes, and the intended usage of some of them is pretty obvious. When it comes to classifying the foot structure of birds, there are several characteristics that are noted, but the defining factor is the orientation of the digits (toes). Birds generally have four toes.
One term you might not know that’s important is the hallux - this is the innermost (“first”) digit of the foot, homologous to the big toe in humans. In birds, it often points backwards. It’s sometimes very well-developed (such as in perching birds), and sometimes so small it’s almost absent.
Anisodactyly (“unequal digit”) - This configuration is basically the standard. The three toes face forward, with the hallux facing backwards, so as to let the bird perch. This is present in songbirds and perching birds. Hawks, eagles, and falcons also have this configuration.
Syndactyly (“same digit”) - The third and fourth toes (outer and middle) are united for much of their length, and have a broad sole in common. You can see this in the kingfisher and the bee-eaters.
Zygodactyl (“yoke digit”) - The toes are arranged with digits 2 and 3 facing forwards, and digits 1 and 4 facing backwards. You can see this in parrots, woodpeckers, and roadrunners.
Heterodactyl (“different digit”) - Toes 1 and 2 are facing back, with 3 and 4 facing forward. This is only found in trogons.
Pamprodactyl (“Every digit”) - All four digits face forward, only found in swifts - this is a somewhat contested classification, as it’s believed that no birds use this as a primary configuration, even if swifts have been observed using it during their rare landings.
There are other useful terms when classifying birds by their foot structure, that have more to do with the type of bird itself, rather than the configuration of its bones. These classifications can include birds with more than one dactylous configuration.
Raptorial - Feet like the raptors and owls. These are strong, deeply cleft, with sturdy talons, meant for grasping and ripping.
Semipalmate - “Half-webbed” feet, where the anterior toes are only partially webbed. The Semipalmated Plover is a bird with these feet.
Totipalmate - “Fully webbed” feet, with all four toes united by one web - these are found on birds like cormorants.
Palmate - “Webbed” feet. These are your “basic” webbed feet - the three front toes are united, like in gulls and ducks. The fourth digit is not connected to the web.
Lobate - A swimming foot with a series of lobes along the toes. Found in birds like grebes.
Tableau Elementaire de l’Histoire Naturelle des Animaux par Georges Cuvier. 1798.