As long as you're out there observing your chickens, go ahead and deliver some discipline when you see this behavior. Chickens are used to the alpha member of the flock giving them a sharp peck on their back when they are engaging in disruptive behavior. It usually works when you do the same thing.
As long as you know who the culprit is, be vigilant and ready to swoop in a poke her sharply as soon as she makes a move to go for a butt feather. The sooner you can do this, the more powerful the message, and the quicker you will be able to stop this behavior.
Also, assess your run. Does it have adequate space for the youngsters to evade bullying tactics? Are there enough extra perches and high places for the chicks to rest without being harassed? Making sure there are these escapes will lessen the opportunity for feather picking.
Adding a flock scratch block will provide all day entertainment to further decrease boredom, one of the leading causes of feather picking besides protein issues. And if patches of skin are showing, painting them with Blu-kote will help disguise them, decreasing further picking.
