I do not know if the feathers are being picked or what but I suspect they are. I only have 4 hens and 3 out of the 4 have bare butts. I suspect the one hen is the one picking the others because she is the boss in the coop. I let them out for several hours every evening. If it were mites, wouldn't all of them have the bare butts? All 3 have had bare butts since last summer and the feathers still have not grown back. I do not know what to do about this. I live in a warm climate so not worried about them freezing and they do not act like anything is wrong. Any suggestions?