- A bird that is a dores – it hunts its prey – is not a kosher bird
- A bird that has an extra toe, a crop, and a gizzard that can be peeled is a kosher bird.
Although we no longer rely on these indicators (see the Rema, Shulḥan Arukh Yoreh De’a 82:3), it is clear that in the time of the Gemara it was commonplace to rely on them to determine whether a given bird was permissible. The Gemara on today’s daf, for example, relates that a type of songbird, the zarzir – identified as a type of starling – was eaten by the people of Kefar Temarta since it has a crop, and that the senunit levana – identified as a type of swallow (hirundo rustica) or martin (delichon urbica) – was eaten by the people of the upper Galilee because it has a gizzard that can be peeled. In both of these cases Rabbi Eliezer argued that it was forbidden since these are types of crows that are listed in the Torah among non-kosher birds.
The Gemara also offers a list compiled by Rav Asi of eight birds regarding which there is a doubt, specifically the ḥuva, ḥuga, suga, harnuga, tushelemi, marda, kuḥilna, and bar nappaḥa. The Gemara explains that Rav Asi’s doubt stems from the fact that in these birds the gizzard can only be peeled with a knife. Although the Gemara relates the case of a duck belonging to Mar Shmuel, where the gizzard could not be peeled, so it was left in the sun, and as soon as it became soft it peeled easily, the Gemara explains that in that case as soon as it became soft it peeled easily with the hand, but here even after it had been softened it could only be peeled with a knife.
In an attempt to identify the birds in Rav Asi’s list, researchers have gone so far as to check which birds have such gizzards.