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Generally that's from other birds feather picking. Sometimes it's from aggression and other times it's from a protein deficiency. Since only one is getting plucked I would guess she's a target. Reasons for it can be a protein deficiency, or crowding and boredom.
I will check the others for pests too, just to be sure. Thanks for your advice!One other thought. Check your other 3 chickens for mites/lice. Some chickens deal with pests by dust bathing so aggressively they strip their feathers right off. It takes care of the pests, but costs them their feathers.
If no other birds have pests, I would guess feather picking as well.