Birds

also in some parts resemble the above mentioned animals;
that is to say, they have in all cases a head, a neck, a back, a
belly, and what is analogous to the chest. The bird is remarkable
among animals as having two feet, like man; only, by the way, it bends
them backwards as quadrupeds bend their hind legs, as was noticed
previously. It has neither hands nor front feet, but wings-an
exceptional structure as compared with other animals. Its
haunch-bone is long, like a thigh, and is attached to the body as
far as the middle of the belly; so like to a thigh is it that when
viewed separately it looks like a real one, while the real thigh is
a separate structure betwixt it and the shin. Of all birds those
that have crooked talons have the biggest thighs and the strongest
breasts. All birds are furnished with many claws, and all have the
toes separated more or less asunder; that is to say, in the greater
part the toes are clearly distinct from one another, for even the
swimming birds, although they are web-footed, have still their claws
fully articulated and distinctly differentiated from one another.
Birds that fly high in air are in all cases four-toed: that is, the
greater part have three toes in front and one behind in place of a
heel; some few have two in front and two behind, as the wryneck.