Several of my 12 red sex-link chickens are missing feathers from their back at the base of their tail. Why are they like this? They appear to be healthy otherwise and I don't see any evidence of mites or anything like that. We keep their coop very dry and dusted with DE. At first we thought it could be this East Texas heat we've been having lately, but the heatwave has mostly broken and the chickens seem to be worse. Only one of them seems to be noticeably red. Could this be self-inflicted? Any ideas? Here are pictures of the worst 2. I don't think all of them are like this, only about 4 or 5 maybe. I haven't seen anything I would call aggessiveness between them. Some of the chickens have picked at the others bare spots, but it seemed to be more curiousness than them being mean. Any help would be appreciated!

