Chullin, Chapter 3, Mishnah 4. Having listed the injuries that render a bird a tereifah, the Mishnah now turns the page and presents the opposite list: wounds that look serious, sometimes even gruesome, yet are not life-threatening. A bird carrying one of these injuries is fully kosher, provided of course that the shechitah was performed properly.
The Mishnah introduces the list with the words "Ve'elu kesheirot ba'of", and these are the cases in which a bird remains fit.
A punctured or split windpipe
"Nikvah hagargeret o shenisdekah": if the windpipe was pierced with a hole, or if it was split along its length. Neither of these is counted as a fatal defect in a bird, and the bird is kosher.
The weasel's bite
"Hikatah chuldah al roshah makom she'eino oseh otah tereifah": a chuldah (weasel) struck at the bird's head, but at a spot where such a wound carries no tereifah status. Why should a strike to the head worry us at all? Because the teeth may drive down as far as the membrane that sheathes the brain and perforate it. Our case, however, involves a location on the head from which the teeth could never have reached that depth. The membrane was never breached, and the bird stays fit.
The crop
"Nikvah hazefek": if the crop was punctured. The crop is the pouch in which the food collects after the bird swallows it and before it moves on to be digested. A hole there does not render the bird a tereifah.
"Rebbi omer afilu nital": according to Rebbi, the bird stays kosher even when the crop has been taken away altogether, as long as it can still handle its food. What Rebbi cares about is whether nourishment can pass from the esophagus onward and reach the gizzard. As long as that process still works, there is no grounds to call the bird a tereifah.
Intestines that emerged intact
"Yatze'u bnei me'ehah velo nikvu": if some injury opened the bird's belly and the intestines came out, but they were not punctured, the bird is kosher. The displacement alone is not what disqualifies; a perforation would be.
Broken wings and legs, stripped feathers
"Nishtabru gapehah", the wings suffered a break; "nishtabru raglehah", the legs suffered a break; "nimretu kenafehah", the outer plumage was pulled out. Those outer feathers are not something the bird needs in order to be shielded, and its life continues without them. Every one of these birds is kosher.
Rebbi Yehudah on the down
"Rebbi Yehudah omer im nitlah hanotzah pesulah": Rebbi Yehudah adds a limit. If the down was removed, the bird is unfit. The notzah is the soft, fluffy layer of feathers that lies directly against the bird's skin, and the bird truly relies on it for protection. Stripping it away is comparable to removing an animal's hide, which renders the animal a tereifah, and Rebbi Yehudah applies that same reasoning here.