The Torah's list of the non-kosher birds includes some modification, "along with its kind." This is true of the "orev," a crow. And also the white "s'nunit." Which seems to have one sign that it's a kosher bird, but Rabbi Eliezer takes a more stringent view, saying that those who treat the bird as kosher (and eat it) will pay for that. Note also the distinction between a white-bellied white snunit and a yellow-bellied white snunit. Plus, a sign of a kosher bird (like a gizzard that can be peeled) does not present in the same way in every bird that has it.