Hullin, Chapter Three, Mishnah Six
Our mishnah provides the characteristics by which it can be determined whether a bird is clean (kosher) or unclean (tref, in modern terminology).
1) The characteristics of cattle and of wild animals are stated in the Torah.
2) The characteristics of birds are not stated, but the sages said: every bird that seizes its prey is unclean.
3) Every bird that has an extra toe, or a crop and a gizzard that can be peeled, is clean.
4) Rabbi Elazar bar Zadok says: every bird that parts its toes is unclean.
Section one: Leviticus 11 teaches characteristics, split hooves and cud-chewing, that distinguish clean beasts or wild animals (such as a deer) from unclean beasts, such as the pig and camel that cannot be eaten.
Section two: The Torah does not list the characteristics of birds that are permitted or forbidden. Rather, it provides a list of names of birds that are forbidden. The rabbis had to fill in the gap and provide a general characteristic: any bird that seizes its prey, be it other birds or small animals, is unclean and not kosher. Just as the animals that we eat are peaceful animals that eat only vegetation, so too the birds that we eat do not eat other birds (although they may eat worms and insects, and not just vegetation).
Section three: If the bird is not one that seizes its prey, then there are a couple of signs that can be used to determine whether it is pure. These would be necessary in case new birds are discovered and one does not know whether they can be eaten.
If the bird has an extra toe on the back part of its foot, it is clean and can be eaten. If its crop (explained in mishnah four) or gizzard can be peeled from the flesh by hand, then it is also clean. Commentators debate whether the bird needs to have all of these characteristics to be kosher, or whether one is sufficient.
Section four: Rabbi Elazar bar Zadok adds a sign to determine when a bird is impure. If when stood on a rope it splits its toes, two in front and two in back, then it is unclean. This is a sign that the bird is a predator.