Chullin 61 - The Signs of Kosher Birds
The signs of kosher birds were not stated in the Torah. Instead, the Torah lists twenty-four types of birds that are not kosher, with the implication that all others are kosher. The Sages, however, said, "Any bird that claws is nonkosher, but any bird that has an added toe, has a crop, and whose gizzard can be peeled, is kosher." Rabbi Elazar the son of Rabbi Tzadok saiys, "Any bird that part the toes of its feet when placed on a twig, so as to grab it better, is nonkosher."
Thus, there should be thousands of bird types that are kosher. However, later, and perhaps already in the times of writing down the Talmud, that is, around sixth century, the people became less knowledgeable in the names of the birds, with the result that they no longer knew with certainly which birds are nonkosher. The signs above, given by the Sages, also cannot be ascertained with ease. The result of this is that only those few birds that are known by tradition to be kosher can be eaten, while the rest cannot.
Art: Joseph Crawhall - The Aviary