When one community has a clear tradition that a particular bird is kosher, other communities may rely on that tradition as well, provided the tradition is known to be reliable and the only reason it was preserved there is that this bird was found in that locale throughout the generations.
If, however, the communities that refrained from eating the bird had a reason to prohibit it, their custom remains binding. This may be because the bird’s features raised doubt; because they believed that the community that ate it had not transmitted the tradition carefully enough; or because the bird resembled non-kosher birds, raising concern that people who ate it might come to eat non-kosher birds as well. In such cases, those communities must continue their custom not to eat that bird (Shakh YD 82:11, based on Rosh and Rashba). Members of the community that did eat it may continue their custom, provided it was practiced with the approval of their rabbis.
Turkey raised a well-known halakhic question. Since we eat only birds with a tradition of kashrut going back to the time of the Mishna, how could turkey be permitted? It originated in the Americas, which became known to Europeans only in 1492, so no such ancient tradition could have existed. Although turkey has all the kosher features enumerated by the Sages, some rabbis therefore refrained from eating it because there was no tradition concerning it.
In practice, however, turkey was ruled kosher, and Jews customarily eat it (Knesset ha-Gedola, YD 82:27). Apparently, when rabbinic authorities first encountered turkey, they did not regard it as a new species requiring its own tradition, but simply as another breed of chicken. Had the question first come before other authorities, they might have inclined toward stringency and treated turkey as a separate species. But once it had been ruled permitted and the practice spread through many communities, turkey became permitted for all Jews.
Others explain that the requirement of a tradition applies only to an uncommon bird. In such cases, even if the bird has the kosher features, we are still concerned that it may sometimes attack prey with its talons, and that this behavior has not been observed because the bird is not commonly found among us. But when a bird is found among us by the thousands, and everyone knows that it is not dores, it may be eaten on the basis of the kosher features enumerated by the Sages (Arugat ha-Bosem 16; Meishiv Davar 2:22). This, they argue, is the reason for the accepted practice regarding turkey.
