Zevachim 70 - Ritual Impurity of Birds According to Rabbi Yehudah
Rabbi Yehudah has said that if a Kohen did a melikah to a bird and then the bird was found to be a "trefah" - possessing a defect that would surely cause its death anyway - that bird imparts ritual impurity. Why does he say so? After all, Rabbi Meir compared this to the slaughter of animals, where the slaughter does remove the impurity.
Rabbi Yehudah found a verse which he expounded. In regard to the carcass of a kosher species of bird the Torah says " died by itself or was torn by wild animals ". Why is the word "torn" mentioned at all? If a torn animal can survive for a year, then it does not transmit impurity until it actually dies. And if it cannot survive a year, then it is already like dead, and this is the case of "died by itself," and it imparts impurity. Either way, we already know the answer.
Therefore it must be that the extra mention of the word "torn" teaches us that if it was torn, then even if it was slaughtered correctly, this does not remove its ritual impurity, and one who eats it becomes impure.
Art: William Gowe Ferguson - Still Life Of Dead Birds