Comp. Mishnah, Treatise Chullin, iii. 6.—The characteristics of the
clean and the unclean cattle and beasts are given in the Pentateuch,
but the characteristics of the clean or the unclean birds are not
mentioned in the Law. Our Sages, however, declared every bird of
prey—or every bird that seizes its food with its claws, and lifts it up
from the ground before eating it (Rashi) to be an unclean bird; while
all birds are clean that have a projecting claw (אצבע יתרה a claw
longer than the rest) and a crop (זפק), and whose stomach has a
membrane that can easily be peeled off (קרקבנו נקלף). Another rule is
this: Birds that dwell and associate with unclean birds are unclean.
“Not without reason does the starling go to the raven: they are of the
same species” (Babyl. Talm., Chullin 65b). The sentence serves also as
a moral lesson (ibid., Baba Kama 92a). At present, however, only those
birds are killed for food which are known traditionally to have always
been considered as “clean birds” (Maimonides, Mishneh-torah, Hilchoth
maachaloth asuroth, i. 14–20; Tur Joredeah, chap. lxxxii.).