This url has a discussion that is far too technical for me to understand - essentially, my impression was that kosher birds are named specifically in the Torah, and while it is possible to draw certain commonalities between which are kosher and which aren’t (i.e. a newly discovered raptor would not be kosher even if it is not specifically named in the Torah) I don’t see how a bird that wasn’t known when the Torah came to be could be considered kosher.
My question hinges on exactly what makes a bird kosher (it’s more complicated than is the case with mammals or fish) and how that’s established with new birds - are ones sufficiently similar to birds known to be kosher or treyf assigned to the same category? Or what?
Can anyone explain the general principles determining whether a bird is kosher or not, and explore why turkey is considered kosher?
Because your first impression is incorrect: it is the birds that are not Kosher that are named specifically in the Torah.
Although not all the non-Kosher ones are definitively identifiable today, there is clear tradition in regard to certain birds which are kosher (mainly due to having been continuously eaten), and to be certain that what they are eating is not one of the forbidden ones (which there is slight ambiguity about), we generally stuck to those. This list (I think) pretty much consists of chickens, ducks, geese, pigeons, doves, pheasants and peafowl.
When the turkey was discovered, there was major controversy amongst the Rabbis of Europe as to whether or not it could be eaten…not so much for fear that it was genuinely non-Kosher (because although we cannot definitively identify the non-Kosher birds by species, we have traditional general descriptions, none of which apply to the turkey), but for fear of breaking with precedent by allowing a bird not traditionally identified as Kosher to be added to the list. After much debate, most Rabbis decided that the turkey was similar enough to the chicken that it could be considered within that category of bird and not be breaking with tradition. There have been some holdouts; I believe that there are a number of Jewish communities who will not eat turkey to this day.