In this teshuva from 1884, the Netziv deals with the question of the kashrut status of a bird that had been thought to be some type of a goose, but upon further inspection was sufficiently different in its physical characteristics that it was supposed that it could be an entirely different species. Since birds require a mesorah in order to be declared acceptable to eat, this bird would potentially be deemed non-kosher if it would be found to be a new species altogether. Further complicating the picture is the fact that people had been eating this bird on the assumption that it was essentially a goose, and only now did someone come forward with this question.
The Neztiv begins by noting that unlike other "new" birds which possessed the signs of a kosher bird mentioned in the mishna in Chullin 60 and which had the testimony of someone that it was eaten by Jews elsewhere, the bird in question here was significantly different from other kosher birds, and was not known to be eaten elsewhere. He then mentions the hybridization test, which is mentioned in the Gemara in Bechorot 7 and which claims that kosher and non-kosher species cannot produce live young. Thus, if two different animals can produce children, and one is known to be kosher, then the other one should be kosher as well. While the Netziv feels that this rule may not be 100 percent accurate, he does qualify it by saying that if a bird from one species will mate with a bird from another one in the presence of birds of his own species then that is an indication that they are all of the same species.
However, the Netziv concludes that this is all academic, and would only be relevant if the question had been brought before people had begun eating this bird. However, once the bird has been eaten on a regular basis, only proof that it is not kosher (such as proof that it is a predator) would disqualify it from being an acceptable bird to eat.
[While it is not clear what bird the Netziv is discussing, this issue is one of the key issues involved in the eating of turkey, which, as a New World bird lacked a mesorah for its kashrut status, but which was ultimately subsumed under the mesorah of chickens, ducks, and geese - and once it gained acceptance it never lost it.]