I have 11 hens - about half of them have red butts with few or no feathers. I have read old posts and cleaned the whole coop top to bottom, dusted with DE (which I do anyway), cleaned nesting boxes, checked for lice & mites and found none. Everyone seems fine, laying fine, I don't see any bullies - what else??? Their butts don't look bad, just bright pink - no sores - no swelling - I am baffled. Any ideas?