Can it cause a clogged butt? I think my old english game bantam hen has mites. I brought her inside to clean her butt and found super tiny black bugs crawling on my hands.
Luckily I haven't had to deal with lice or mites yet, but here is a link I keep on hand with some great information about identification of and treatment for both of these pests. Hopefully once you clean her up and eliminate the pests, she'll have no more issues. Hope this helps, and best of luck to you with this