This may seem like a pretty trivial difference, but in fact it is an outward expression of a host of other distinct differences between these classes. It is still in use today. To say that birds can be identified as two-legged creatures makes Man a bird. To call birds all animals with wings makes bats and flies birds. To say that feathered creatures defines birds hits the mark.
All birds have feathers, and no other creatures do. . .
Ray worked on animals as well, and was strongly influenced by the bible. The later includes tigers, anteaters, etc. and would be unclean. Of those with hoofs, the further division is cloven hoofs. Cloven hoofs are two hoofs per foot. One or three or more are non-cloven hoofs.