The Torah establishes that only a bird classified as pure may be eaten (Devarim 14:11-20), and the Gemara sets out physical signs of a kosher bird — not clawing its prey, having an extra digit, a crop, and a peelable gizzard — while noting that these signs alone do not suffice for certainty, since even a bird thought pure was later found to be a predator (Chullin 59a:6, Chullin 61a:1-2).
Because physical signs can mislead, the Rambam, the Beit Yosef, and the Shulchan Arukh all rule that a bird is eaten only on the strength of a received mesorah — a tradition handed down that this species is kosher — and that signs alone, absent such a tradition, never sufficed to permit a bird (Mishneh Torah, Forbidden Foods 1:19, Beit Yosef, Yoreh De'ah 82:7, Shulchan Arukh, Yoreh De'ah 82:2-3). The Arukh HaShulchan and the Kaf HaChaim reaffirm this principle, stressing that without a mesorah even full physical signs do not permit a bird, and that a mesorah itself is voided if the bird is ever observed clawing prey (Arukh HaShulchan, Yoreh De'ah 82:1-20, Kaf HaChayim on Shulchan Arukh, Yoreh De'ah 82:1-20).
This mesorah requirement is exactly what generated the historical difficulty with newly-encountered birds, including fowl from the Americas: the Darkhei Teshuvah records disputes over various new birds (the Pererl Hiner, the Gretzki Hener, the Indishe Hener) precisely because they lacked an ancestral mesorah even though they displayed the physical signs of purity, with authorities divided over whether a mesorah from one region could be relied upon elsewhere (Darkhei Teshuvah, Yoreh Deah 82:46, Darkhei Teshuvah, Yoreh Deah 82:28, Darkhei Teshuvah, Yoreh Deah 82:26, Darkhei Teshuvah, Yoreh Deah 82:43). The Shoel uMeshiv likewise discusses a newly-arrived fowl with full signs of purity that was nevertheless eaten without hesitation in the Land of Israel and elsewhere, ultimately ruling leniently (Shoel uMeshiv Mahadura III 1:149), and the Noda BiYehudah treats an analogous problem of relying on local testimony absent an established tradition (Noda BiYehudah II, Yoreh Deah 28:3-4).
Applied directly to turkey, Rav Eliezer Melamed states that turkey's origin in the Americas, discovered only in 1492, meant it had no mesorah from the time of the Mishnah, and some rabbis therefore hesitated to eat it despite its possessing all the signs of purity; nonetheless the practical ruling that emerged, and the practice of the Jewish people, is that turkey is kosher, apparently because the early rabbis who encountered it understood it not as a wholly new species but as another breed of fowl, for which no fresh mesorah was required (Peninei Halakhah, Kashrut 17:6).
Responsa Benei Banim addresses the specific practice of eating turkey on Thanksgiving, ruling that doing so is not prohibited under the principle against following gentile customs, since the holiday's rationale of giving thanks is itself a Torah-rooted idea belonging to all citizens and not a specifically Christian one, so eating turkey as a token of that gratitude is permitted (Responsa Benei Banim, Volume III 37:5).
Iggerot Moshe likewise treats participation in a Thanksgiving turkey meal, holding that it is permitted when the day is understood as a civic commemoration rather than a religious observance, and that eating turkey as part of a voluntary celebration is permitted so long as the day is not treated as obligatory or as a mitzvah (Iggerot Moshe, Yoreh Deah 4:11).
Maayan Omer, discussing a related turkey kashrut question involving turkey necks, notes that the matter requires careful factual investigation and leaves some uncertainty, while observing that packaged turkey necks later came to be sold under a local rabbinate's kosher-for-Passover certification (Maayan Omer 3:2:44).
The egg-sign rules of the Shulchan Arukh, requiring identification of the source bird before relying on an egg's shape, form part of the same broader system of signs and mesorah governing bird kashrut, though they address eggs rather than the fowl itself (Shulchan Arukh, Yoreh De'ah 86:1-2).
