this is exactly the problem I have with two of my hens - and none of the other seven or the roo - and the two who have it are mother and daughter! The young hen had a patch of what looked like broken-off feathers there for a while but quite suddenly (within the last two days) it's down to bare skin. Mother's back has been like that for several months. Coop and hens all treated regularly for mites. Everyone else looks great. Both are daily layers, otherwise seem healthy (tho the older hen is a little prone to poopy butt). Curious if you have determined for sure what you are dealing with.I've been dousing Mama's back with iodine which she hates and I can't tell that it is helping at all.