The Gemara says that the signs for a bird to be kosher aren't explicit in the Torah - except that the Gemara also says that the signs for a bird to be not-kosher are explicit in the Torah -- via the "nesher" -- so the signs are identifiable. But does the nesher provide rules of thumb or is it specifically and only itself as a non-kosher bird? Plus, naming some non-kosher birds that are listed specifically, but that don't have all 4 signs of being a non-kosher bird that a nesher has. But these two named birds become significant in terms of representing which single kosher signs render the birds into examples for the kosher-ness of other birds too.