This is a topic where there are no easy, pat answers. Thinking that feather picking is a result of not enough protein is naive and simplistic. Until you've wrestled with this problem in your flock, you really can have no realistic idea of what a flock keeper with a feather picker is dealing with. Assuming it happens because of crowded conditions is also risking confusing coincidence with cause. Overcrowding has never been an issue in my flock nor has lack of protein.
I've been wrestling with this problem practically from the time I first got chickens almost ten years ago. The first extreme case I had, though, was a young three-month old EE pullet named Flo. I read that it was lack of adequate protein that caused feather picking, so I started giving her a tablespoon of tuna each day. While she enjoyed her daily treat, it did absolutely nothing to curb her destructive behavior, and probably set her up for gout arthritis as she grew older.
At one point some friends who had poop eating dogs recommended Forco, a pre-biotic supplement for horses, dogs and poultry. It was pricey stuff, but I felt it was worth it if it worked. Flo was not just a feather picker. She was a psychotic feather picker, mowing feathers at a full run, zipping from one victim to the next without even slowing down to rest. The Forco did seem to work, at least for a while, and I thought I had found the miracle cure. I began a thread
https://www.backyardchickens.com/t/697052/i-think-i-found-a-miracle-cure-for-feather-picking on the subject, and it grew to dozens and dozens of pages, people sharing their successes and failures in coping with the problem.
When Flo began her serial picking again, I came to several conclusions. One was that I believe some individuals are hard wired in their brain to pick feathers. Since I had tried everything with Flo, it was natural to come to this conclusion in her case.
Another thing I have observed is that feather picking appears to be seasonal. It also seems to be hormone related, dropping off in the fall, becoming dormant in winter, and in spring, the culprits spring into action again, pardon the pun.
It also seems to affect some breeds more than others, as some of you have already noted. In my flock, the EEs and the Speckled Sussex are the main perpetrators.
There is likely no cure. There really is no sure-fire preventative, either. But there are some things that are effective in the short term, and they may even stop a budding feather picking career before it gets out of hand. Pinless peepers are what I recommend to anyone with this problem, although some chickens can develop "work-arounds". Recently, a new EE pullet was yanking feathers. She had just come into her hormones, and I installed peepers on her for about three weeks, then removed them. She seems to be doing fine now. But I have my eye on her just the same.
I've long been a believer that feather picking is an abhorrent behavior originating in the brain. Most experts recommend extra protein for the feather picker, but I think there may be something else lacking in the diet. B-3 which is niacin, if lacking in an animal's diet, can cause cannibalism. I therefore recommend giving your feather pickers extra B-3 each day.
I find it very interesting that some of you living in the regions of Africa and Australia without seasonal climate changes have experience little or no feather picking in your flocks. I wonder if those in southern California and Florida and Mexico and Hawaii have experience similar lack of picking in their flocks. It would be enlightening to know this.