I'm trying to combat that problem right now. I've isolated the offenders. I plan to reintroduce after the feathers have grown back. Maybe they won't be bullies then.
Feed wise, I've been feeding the higher protein duck & game feed. Kitten food has extremely high amount of protein. The kind I get has 36%. I've added some to their feed. I mixed in the coral shell with their other food. I'm trying to fool them into eating a little more of it. Although, they do still leave it in the bottom of their feeders. Hopefully, this regiment will correct any nutritional deficiencies.
I also made up some natural "chicken butt spray." Right now it has blue food coloring, orange oil, tea tree oil & water. I figure the orange oil has to taste bad, tea tree oil help healing and blue is supposed to keep them from pecking. I noticed one chicken after pecking kept shaking her head. As long as it doesn't kill them and keeps them from pecking, I figure it's working. I'm just experimenting with the CBS so I may change the recipe. I'm sure some of the chickens are laughing at the white ones with bright blue butts.
I've also been giving them hay everyday to give them something else to peck on. I read it can be a sign of bored birds. Never knew I enlisted to be chicken entertainment... Maybe I need to start doing an exercise program in front of the chickens. That should give them something to do.
So far so good. They are growing back their feathers now. The real test will be when I add the perpetrators back to the pen.
Prior to this doing all this, they were killing about 1 chicken a day but it's been a couple of weeks since I have lost one now.
All I can say is I'm trying. And I feel your pain...