A few weeks ago I noticed Esmeralda's feathers on back started looking like this. None of the other chickens have any issues with their feathers and it doesn't seem to bother her. I bought a saddle for her, will this help? I haven't noticed Mercy (the Silver Laced Wyandotte roo) showing her too much attention but yesterday I DID notice he gave her a peck on her back. I looked for mites but haven't noticed any. And, just to complicate matters further, I notice this morning that her bottom, below her vent, is looking a bit featherless. Is this feather picking? Aaaah! I want to help my poor sweet girl.
Just because you don't notice your rooster giving her much attention, doesn't mean he isn't That looks like typical mating wear and tear to me, the saddle will help. If you've got enough hens to keep him occupied, he's probably her favorite. How many hens do you have, anyway? As for the bald bottom, it could be feather picking or it could still be mites. They're awfully small little pests and can be very difficult to see. I would treat her (and everyone else) for them just in case, and give the feathers some time to start coming back in. If they don't, or start to and then suddenly disappear, you've probably got someone picking them off. But, usually if there is a picker, more than one bird will be targeted. Good luck!
Have you considered de-pluming mites? Chickens are the hosts to a wide range of mites as well as lice. The de-pluming mite lives and breeds inside the feather shaft. Also she could be molting.