It looks like the last chicken is molting but I'm not very experienced in this area either. The first chicken's bald spot looks too smooth to have been pulled, it looks like when my baby chicks' butt feathers fell out when they got pasty butt, but this cannot be the cause since there is no dried poop. It could be mites but you ruled that out when you checked them...hmm I read about a problem like this on another site but the help reply wasn't very helpful. Make sure their sleeping accomodations are ok, watch to see if hens are pulling other hens' feathers out, watch to see if they're doing it to themselves, check to make sure they're getting enough nutrients in their food e.t.c. I hope this helped a little bit, though it probably doesn't.