Having clarified issues regarding animals that are considered terefah, i.e. a terminal condition that will lead to the animal’s imminent death, the Mishnayot on today’s daf (=page) turn their attention to conditions that would render kosher birds as terefot. In truth, as Levi points out in a baraita that appears in the Gemara, most of the laws of terefot in birds parallel those of animals that were enumerated in the Mishnayot on daf 42a and daf 54a. Nevertheless, the ones that appear here are mentioned since they clarify rules and regulations that are unique to the situation of birds. So, for example, we find laws regarding the kurkevan (gizzard), which, if pierced, renders the bird a terefah, and the zefek (crop) which will not affect the kosher status of the bird if it is pierced; Rabbi Yehudah haNassi rules that even if it is removed entirely the bird will remain kosher.
The zefek, or crop, is an enlarged part on one or both sides of the esophagus, which is found in many birds, as well as some insects and reptiles. The crop is used to temporarily store food, where harder foods like seeds become softened before continuing down the esophagus to the kurkevan.
Since birds do not have teeth, the process of grinding the food up in preparation for digestion differs from that of animals. The kurkevan or gizzard serves this purpose. This specialized stomach – the ventriculus – is constructed of thick, muscular walls. The bird swallows hard objects, like small rocks or pebbles, and by means of the muscles of the gizzard “chews” and grinds up the food. The structure of the gizzard, as well as the thickness of its muscular walls differs from one bird to another, with the general principle being that the thicker the membrane, the easier it is to peel off.