The following injuries, when it comes to a bird, are not life-threatening, and therefore the bird is still kosher (fit for consumption) if that happened to it properly. The windpipe had a hole in it, or there was a slit along its length. A weasel bit the bird on the head, but in a spot that wouldn’t make it a terefa (unfit). That means it bit the bird in a spot where the teeth wouldn’t have the ability to reach the inner brain membrane and make a hole in it. That wasn’t possible.
The crop is where the food goes after it’s eaten, before it’s digested. Rabbi omer (Rabbi says). It is stated that even if the crop was removed, the bird is kosher as long as it still has the ability to digest the food. According to that view, as long as the food could pass from the food pipe to the crop to the gizzard, things are okay and functioning properly. We wouldn’t say that the animal is a terefa, or that the bird is a terefa.
If the bird’s belly opened up and the intestines came out as a result of some sort of injury, and there was no hole in them; if the wings were broken, the legs were broken, or the outer feathers were removed, these types of feathers are not relied on by the bird for protection, and therefore it could survive without them. In all these cases, the bird is still kosher. The view on the down feathers says that if the down—that means the fluffy feathers that cover the bird’s skin directly—were removed, the bird becomes unfit because it needs those feathers in order to protect itself. It’s like removing the skin of an animal, which makes it a terefa. This is analogous to that case.