Taharos, Chapter 1, Mishnah 2. Masechta Taharos carries a name that is almost ironic, since nearly all of its subject matter is things that are tamei; the title is best read as a euphemism. Its ten perakim take us through neveilah, the tumah of liquids, the halachos concerning amei ha'aratzim, and a surprising amount of expertise about olive oil production. Our presentation follows the shitah of the Bartenura wherever possible.
The perek opened by announcing thirteen matters that distinguish nivlas ha'of hatahor, the carcass of a bird of a permitted species (a chicken rather than a vulture) that died on its own rather than being slaughtered. Note that "tahor" here describes only the species, not the bird's ritual status and not its permissibility. The Torah's discussion of this tumah, in Vayikra 17:15, speaks of one who eats such a carcass, and that shapes everything: unlike the neveilah of a behemah, which conveys tumah by touch and by carrying, nivlas ha'of conveys tumah when it is being eaten, in the throat, with a measure of a kezayis, while a kabeitzah of it conveys food-tumah to other foods it touches. Nine of the thirteen matters were listed in the first mishnah. Our mishnah supplies the remaining four, and they all revolve around the parts of the bird that are not exactly meat.
"Hakenafayim v'hanotzah mitam'os u'metam'os v'lo mitztarfos." The wings and the notzah, the larger plumage, are susceptible to tumah and they also transmit tumah, but they do not combine. That is, if the bird flesh in hand fell short of the required measure, one may not make up the difference with the wings and feathers. The reason is their halachic classification: they are considered a yad, a handle, the part by which one takes hold of the food itself. Masechta Uktzin establishes the rule for a yad: it becomes tamei and it conveys tumah, but it is not counted toward the shiur.
"Rebbi Yishmael omer: hanotzah mitztarefes." Rebbi Yishmael disagrees with respect to the notzah. In his view this plumage is substantial enough that it is not a mere handle but a shomer, a part that guards and protects the food, and a shomer does combine toward the measure.
"Hachartom v'hatzipornayim mitam'in u'metam'in u'mitztarfin." The beak and the claws are susceptible to tumah, they transmit tumah, and they do count toward the shiur, because here everyone agrees that their status is that of a shomer rather than a handle.
"Rebbi Yosi omer: af rashei agapayim v'rosh hazanav mitztarfin." Rebbi Yosi broadens the list still further: the far ends of the wings, along with the very end of the tail, also join the flesh in making up the shiur. He gives his reason: "sheken manichin b'pitumos." Normally these extremities are cut away as worthless, but with fattened birds they are left attached, and being left on is enough to give them the standing of the bird itself.
With these four additions, the count of thirteen distinguishing features of nivlas ha'of hatahor is complete, and the masechta is ready to move forward.