My 5 week old RIR has lost her feathers on her back up to the stem part (I haven't looked at what the technical name is yet) and I don't know what the cause of it is or what I should do. I have 13 hens brooding in a rabbit cage right now. The RIR seems to act fine. I haven't seen any pecking of the other chickens and there isn't any bleeding on her back, just the feathers are gone. Does anyone know what this could be or how I can help it?