Chullin, Chapter 3, Mishnah 3. The opening mishnayot of this chapter listed the eighteen conditions that render an animal a tereifah, a creature so damaged that it cannot survive and therefore cannot be made kosher by shechitah. Now the Mishnah turns to birds. A bird is built differently from a beast, and so the list of fatal injuries is its own list. Our mishnah opens with the words "Ve'eilu tereifot ba'of": and these are the injuries that render a bird a tereifah.
The neck: esophagus and windpipe
"Nikvat haveshet": a bird whose food pipe has been punctured. Any hole in the esophagus is fatal.
"Pesukat hagargeret": the trachea cut through its breadth, and only where the incision has passed the midpoint. On this point a fowl is governed by precisely the same rule as a beast. A different conclusion might well have suggested itself. Valid shechitah in a bird demands the severing of just one pipe, trachea or gullet, as the earlier mishnayot established, so one could have supposed that a bird's trachea is the frailer of its organs, and that even a slight perforation there would already be fatal. Kama mashma lan: the Mishnah rules out that supposition. Anything short of a breadthwise cut extending past the halfway mark leaves the bird fit.
The bite of a weasel on the head
"Hikkatah chuldah al roshah": a weasel struck the bird on the head. The Mishnah adds the qualification "makom she'oseh otah tereifah", in a location where a bite of this sort is disqualifying, namely upon the skull. What worries us is that the creature's teeth may have driven right through the bone and pierced the membrane that sheathes the brain.
Because the damage is hidden, the bird is examined after the shechitah. A finger is inserted into the bird's mouth and pressed upward against the palate. If brain matter emerges through the fracture in the skull, we assume the brain itself was breached and the bird is a tereifah. Here too there is a chiddush. One might have thought that in a bird a fractured skull alone is enough to disqualify it. The Mishnah teaches that unless the membrane of the brain has been compromised and its matter emerges, the bird is not a tereifah.
The gizzard and the intestines
"Nikvah hakurkevan": a hole in the gizzard. The gizzard is the sac enclosing the muscular tissue that mills the bird's food before it moves on into the intestines. Disqualification here requires perforation of both layers, the muscle together with the sac surrounding it, and the two openings must lie opposite one another. Should the holes fail to line up, or should the muscle alone have been pierced, the bird remains permitted.
"Nikevu hadakin": holes in the small intestines. Every injury enumerated to this point leaves the bird a tereifah.
A bird that fell into the fire
"Nafelah la'ur venechmeru benei me'eha": the bird dropped into fire and the organs within it were singed, the heat having reached through the skin and harmed what lies beneath it. "Im yerukim, pesulin": those organs are naturally red, so once they have turned green, the discoloration proves that the tissue is spoiled and will in time break down and perforate. The bird is a tereifah immediately. "Im adumim, kesheirim": should they retain their natural redness with no damage apparent, the bird is fit.
Crushing and impact
The final three cases of the mishnah deal with a bird that suffered a violent impact, along the lines of the animal that fell from a roof in the first mishnah of the chapter. "Derasah": someone trod on the bird. "Utrefah bakotel": it was flung against a wall. "O sherirsesatah behemah": or an animal crushed it. "Umefarkeset": and the bird is twitching and flapping, able to move about but unable to walk in its normal manner. In such a state we are concerned that the impact may have inflicted fatal damage on an internal organ, and the bird may well be a tereifah.
"Veshatah me'et le'et, ushchatah, kesheirah": but if the bird lived on for a full me'et le'et, a complete twenty four hours, and was then slaughtered, it is kosher. Its survival tells us that no fatal injury was sustained. This is on condition that the internal organs are examined afterward and found to be intact, and then we rule that the bird is kosher.