Unlike for mammals, the Torah gives not list of simanim ("signs") for
what constitutes a kosher bird, rather, we have a list of non-kosher
birds, many of which we no longer know the identities. The Oral Law,
however, discusses characterstics of all the non-kosher birds (or maybe
it was the kosher ones), listing three characteristics as I recall.
Since the vast majority of birds are kosher (by simple numerical
comparison of the number of birds versus the relatively small list of
treif ones) and the turkey doesn't possess the characteristics common to
a treif bird (not an absolute measure, but rather a "common
denominator"), most poskim ruled that it was kosher. Those of us who
have a mesorah (a tradition) that it is kosher continue to behave
There is even a case I seem to recall learning where a debate similar to
the turkey debate took place in the Gemara with a different bird, and it
gives a better understanding on how such a matter is judged.
Perhaps one of our Yoreh Deah experts could help me find the source and
clarify it. I remember reading a whole discussion on the matter in backs
of one of the Baltimore Parsha sheets.