Rather than giving criteria for identifying kosher birds, the Torah lists the non-kosher species, thereby teaching that all other birds are kosher. As it says:
“Every kosher bird you may eat. But these you shall not eat from among them: the eagle, the vulture, and the osprey; the kite, the falcon, and the buzzard according to its kind; every raven according to its kind; the ostrich, the night hawk, the sea gull, and the hawk according to its kind; the owl, the cormorant, and the barn owl; the pelican, the carrion vulture, and the cormorant; the stork, the heron according to its kind, the hoopoe, and the bat” (Devarim 14:11–18). Similar verses appear in Vayikra 11:13–19.
The Sages said (Ḥullin 63b): “It is revealed and known before the One Who spoke and the world came into being that kosher birds are more numerous than non-kosher birds; therefore, the Torah listed the non-kosher birds,” teaching that all the rest are kosher.
From the Torah’s list, the Sages derived that there are twenty-four types of non-kosher birds. One who can identify all of them by name and by their distinguishing features may treat all other birds as kosher. Over the course of exile, however, this tradition was lost. The Sages therefore provided several identifying features to distinguish kosher birds from non-kosher ones. A bird that is dores, meaning that it seizes and kills prey with its talons, is non-kosher. One indication that a bird is dores is that, when placed on a string, it spreads its toes two to each side.
When it is unclear whether a bird is dores, it may be treated as kosher only if it has all three kosher features: an extra toe, a crop, and a gizzard whose inner membrane can be peeled. If the bird is known not to be dores, then even one of these three features is enough to establish that it is kosher (Ḥullin 59a; Tur and Beit Yosef YD 82:2; Shakh 82:4).
In practice, however, the Rishonim write that these features are no longer relied upon. There were cases in which people used them to identify a bird as kosher, only to discover later that it was dores and therefore non-kosher (see Ḥullin 62b). Therefore, today we eat only birds with a clear tradition of being kosher (Rashi, Rosh, SA and Rema YD 82:2–3).
The kosher birds eaten most widely today are chicken, above all, followed by turkey. Some communities also eat domestic geese, domestic ducks, mute swans, pigeons — including domestic and giant pigeons — turtle doves, sparrows, and quail.
The commentators explain that birds belong to a more refined realm than animals, and therefore most bird species are kosher. Perhaps for that very reason, though, birds are also more distant from us, and the traditions concerning them were lost. In practice, then, only a relatively small number of birds — those for which a clear tradition of kashrut has been preserved — are certified as kosher.
