הלכה על חולין 117:6

From the laws of the commandment is that which they, may their memory be blessed, said (Chullin 59a), "The signs of the the beast and animal are stated from the Torah" - meaning to say they are explicit in the Torah, and that is a hoof completely divided, like the ox and the sheep, and [that] it brings up the cud - "but the signs of the fowl were not stated," since the Torah delineated for us by name all of the impure ones in the world. And since it is so, there is no need to write their signs - as any bird that is found in the world besides them is fit (kosher). And nonetheless since not every man is an expert in all of the species of which the Torah informed us, the Sages informed us of signs through which any man may recognize them and he be able to eat of fowl, even though he not recognize even one of the many impure species that are delineated in Scripture.