Says the Mishna [Zevachim 69b]:
מתני׳ מלק ונמצאת טריפה ר' מאיר אומר אינו מטמא בבית הבליעה
MISHNA: If the kohen pinched the nape of the bird’s neck properly and then it was found to be a treifa, and it was therefore disqualified from being sacrificed and forbidden for consumption by a kohen, Rabbi Meir says: An olive-bulk of its meat does not render one who swallows it ritually impure when it is in the throat, as the pinching prevents it from assuming the status of a carcass.
רבי יהודה אומר מטמא
Rabbi Yehuda says: Its status is like any other carcass of an unslaughtered kosher bird, and its meat renders one who swallows it ritually impure.
א"ר מאיר ק"ו אם נבלת בהמה שמטמאה במגע ובמשא שחיטתה מטהרת טריפתה מטומאתה נבלת העוף שאינו מטמא במגע ובמשא אינו דין שתהא שחיטתו מטהרת טריפתו מטומאתו
Rabbi Meir said: My opinion can be inferred a fortiori. If an animal carcass transmits impurity to a person through touching it and through carrying it, and nevertheless the slaughter of an animal purifies it, even if it is a treifa, from its impurity, i.e., its slaughter prevents it from assuming the impurity status of a carcass, then with regard to a bird carcass, which possesses a lesser degree of impurity, as it does not transmit impurity to a person through touching it and through carrying it, but only through swallowing it, is it not logical that its slaughter should purify it, even if it is a treifa, from its impurity?
מה מצינו בשחיטתו שהיא מכשרתה לאכילה ומטהרת טריפתו מטומאתו אף מליקתו שהיא מכשרתו באכילה תטהר טריפתו מידי טומאתו
And once it is established that slaughter renders a bird that is a treifa pure, it can be inferred that just as we found with regard to its slaughter that it renders a bird fit for consumption and purifies a bird, even if it is a treifa, from its impurity, so too its pinching, which renders a bird offering fit with regard to consumption, should purify it, even if it is a treifa, from its impurity.
רבי יוסי אומר דיה כנבלת בהמה שחיטתה מטהרתה ולא מליקתה:
Rabbi Yosei says: Although one can derive from the case of an animal that slaughter renders even a bird that is a treifa pure, that derivation cannot be extended to pinching. The same restriction that applies to every a fortiori inference, namely, that a halacha derived by means of an a fortiori inference is no more stringent than the source from which it is derived, applies here: It is sufficient for the halachic status of the carcass of a bird that is a treifa to be like that of the carcass of an animal that is a treifa; its slaughter renders it pure, but its pinching does not.
What is R' Yosei's question?! The דין of מליקה is not learned from a קל וחומר. The ק"ו teaches that שחיטה is מטהרת from נבילה. The דין that מליקה is מטהרת is derived from a מה מצינו [see 50b]??! When it comes to a מה מצינו we don't say "דיו"?!
The Sfas Emes asked differently: The דין of דיו teaches us that the למד - what is derived, cannot be more broad than the מלמד - the source. But here where מליקה has the same דין as שחיטה it should not be considered that the למד is more inclusive than the מלמד because מליקה of a bird is classified just like שחיטה.
