Hullin, Chapter Three, Mishnah Four
Todays mishnah teaches what defects do not render birds into a terefah.
And the following [defects] do not render birds terefah:
1) If the windpipe was pierced or cracked lengthwise;
2) If a weasel struck it on the head in such a place as would not render it terefah.
3) If the crop was pierced
a) Rabbi says: even if it was gone.
4) If the innards protruded [from the body] but were not pierced.
5) If its wings were broken, or its legs; or if [the wings] feathers were plucked.
a) Rabbi Judah says: if its down was gone it is invalid.
Section one: This is the same defect that does not cause a beast to be a terefah (see mishnah two).
Section two: The opposite of the section two in yesterdays mishnah.
Section three: Since I dont know exactly what a crop is, I looked it up in Wikipedia. There the crop is defined in the following manner: In a bird’s digestive system, the crop is an expanded, muscular pouch near the gullet or throat. It is a part of the digestive tract, essentially an enlarged part of the esophagus. As with most other organisms that have a crop, the crop is used to temporarily store food. Not all birds have a crop. According to the first opinion, if the crop is pierced, the animal is not a terefah, but if the crop is missing, it is a terefah. Rabbi rules even more leniently and holds that even if the crop is removed, it is not a terefah.
Section four: As long as the innards were not pierced, the bird is not a terefah.
Section five: A broken limb and plucked feathers on the wings do not make the bird into a terefah. Rabbi Judah holds, that if the birds soft cover of down was removed, it is a terefah.