Take a toenail clippers and nip the beaks of the peckers. If the 'underchick' has an open wound, you'll have to separate her, as they'll peck her to death. Chickens love to peck blood - they'd eventually kill her. When you clip the beak, just take a tiny bit off the TOP beak only, so it doesn't hang over the bottom. Then look at it straight on - you should see a little curved open area. Feathers slip thru a clipped beak. Don't clip too far, or it will bleed. If you do it right, it will grow back and it doesn't hurt. But this will stop feather picking, as the feathers will slip thru the beak. Later, if this bad habit starts again, check the beak and clip again. I use this method, and I only have to do it once and the plucking stops. Hope it works for you. It may or may not be because you have a roo - sometimes dominant hens do it, too.
