It could be mites, but I'll bet it's feather picking by other birds. I had several bright, flaming, red butts in my birds due to that particular problem. Whenever chicken skin is exposed to the elements it turns bright red whether it is on their butts or elsewhere. The location is indicative of feather picking, though. Butts and lower backs are target areas. You should take some time to watch your flock. I did that, and within an hour I had pinpointed a culprit who was walking up behind birds and just yanking out butt feathers by the mouthful. I'm guessing you have the same thing going on.
Good luck.