Rabbi Jeremy Gordon discusses Why turkey gave some rabbis a headache.
There is a list of birds known in the ancient Near East which are forbidden, but what of a bird unknown to the world of the Torah and Talmud? Rabbinic argument has split between those, such as the Rosh, who consider that there are simanim that, if they can be observed on a newly discovered bird, would allow that bird to be deemed kosher; and those, such as the Shach, who insist a bird can only be considered kosher if there is a clear tradition, or masorah, that such birds were always considered kosher — which would seem to render the turkey forbidden.
...The tale was made more complicated by the initial, erroneous, assumption that turkeys were nothing more than larger (and “Indian”, hence the Hebrew term hodu for turkey) versions of the European chicken. By the time this zoological confusion had been clarified, Jews had been eating turkey for some time and, in a coup of legal realpolitik, virtually all halachic authorities deemed turkey acceptable since it had both the simanim and, by this time at least, a demonstrated masorah.
Read more: http://www.thejc.com/judaism/judaism-features/113223/why-turkey-gave-some-rabbis-a-headache