Could it be mites or lice causing this. I would check for that cause if they are bad on her, it could be causing the feathers to come out, and probably would make her sick or swollen or any other secondary symptoms. Have you noticed her scratching a lot? The mites can be hard to see they are very small, there is one type that are white and another type that are even harder to see because they are tiny and red I think, and they come out at night. You can go to the search bar and type in mites and lice treatments and there may be a link to the different types of these kinds of pests, or type in pests? Anyway I would check that first. Hopefully someone else more knowledgable will answer you. Good Luck.