This is a universal problem. A very unhelpful common wisdom insists feather picking is diet related and all you need to do is increase protein you feed your flock. This is very far off target, and feeding extra protein just because you believe this CW that protein will fix the problem can end up causing serious health issues and early death. This is something I have personal experience with. I believe feeding extra protein for extended periods is dangerous and I will not recommend it.
The actual truth is feather picking has multiple causes, and most are not easily solved. In my flock, I have eighteen hens and all but one will munch on my two roosters' neck and saddle feathers. The roosters actually encourage the hens to do this by pushing their necks under the hens' beaks.
Three of these hens are wearing pinless peepers because they munch the feathers of the other hens. These hens appear to have a compulsion to pluck feathers, and I believe they have mis-wired brains that cause them to fixate on the feathers because they aren't really seeing the chicken behind the feathers. The peepers do seem to help.
To protect the roosters, I house them separately in their own coop section and their own run. The roosters free range separately from the hens. This is the only way the roosters get to keep their feathers.
