Feather picking, which is what you're dealing with here, is a behavior issue. Many folks will jump in and advise checking the protein levels of your feed, measuring the living spaces for crowding problems, etc. But no one really can say why some chickens pick feathers off others' hides.
I'm going to tell you what to do to stop it.
If you're certain this is occurring at roosting time, you have the first step licked - catching them in the act. Watch carefully and the second you see the culprit go for a feather from her victim's neck, HOLLER! Go, "No!" loudly and sharply. Watch. Wait. As soon as she starts to do it again, Go, "No!" You will see her pause, look up at you, and probably start in for a third time.
Again, Holler, "No!" It's been my experience that after getting hollered at a third time, the behavior ceases.
It's my theory that this holler therapy interrupts the behavior and each time, as it interrupts the behavior, it's also changing something in the brain of the perpetrator. I've been trying this out on my own flock, which has had its share of serious feather-picking, and by golly, it's working.
In your case, I would monitor roosting time a couple more nights to make sure if additional therapy is needed. Just repeat the hollering of "NO!" each time you see this behavior, including during the day. No need to station yourself in the run for hours. Simply be watchful for the bad behavior and intervene the second you witness it, following through until it stops
Report back and tell us how it goes.
