There are many causes of feather picking, and many we really aren't sure of. The sad thing is there's no real cure. As with other chicken behaviors, feather picking might begin due to a specific cause such as lack of sufficient protein, but it can then proceed as a habit long after the cause has been remedied.
Do introduce a source of additional protein if you feel the flock may not be getting enough. But you might try something that could stop the habit from developing into a long term problem.
I've discovered that if I sit for a few minutes in the run as a quiet observer and just watch the hens as they interact, I'm in a perfect position to interrupt bad behavior.
This week I discovered the four Sussex in the secluded run in the rear have a picker that is fraying the saddle feathers that have just grown in after molt on sweet little Judy, the most docile one of the four. As soon as I see Sylvia, who is the culprit, start drilling Judy's back, I holler, "NO!" And each time she begins again, I holler, "NO!"
During one session, Sylvia appeared to ignore me and kept right on with her predations. So I got down on her level, on my hands and knees, and yelled, 'NO!" right in her ear just as she was going for Judy's feathers. That did it.
She stopped. And the behavior is appearing to taper off as she has associated being hollered at with her compulsion to pick feathers. Something seems to be wiring itself differently in her brain, interrupting her compulsion.
This approach has already succeeded with another serial feather picker in the main run, so it's already got a proven track record of success. You don't have to be present all the time around the chickens for it to work. Simply spend a little bit of time observing and interrupting the behavior. After a while, it seems to change things.
You have nothing to lose by trying it.