So yeah. This started happening a while back. One chicken has had this bald patch under her vent for a long time. Her butt is also a bit swollen but it seems like it's under control. It was always there, didn't grow any larger and she's healthy otherwise. The other chicken just recently started losing her butt fluff. The skin looks normal to me except for a few black spots which are where the feathers used to be. No idea what these are. I hope a picture will help. Both chickens are completely happy otherwise. Last year we had a huge invasion of red mites in the coop. The walls were literally red with blood. Now there are still mites here and there but nowhere near like last year. I'm in the middle of solving this problem but I figured it could be a clue for my main problem. I doubt it however since I have 18 chickens and only 2 are having the balding problem. Hope to hear your opinions soon!