It could be the other hens picking her feathers out. Or it could be mites or lice. If there is any bleeding, I would use Blu-Cote to cover that or she will get picked even more. Examine her skin in that area very closely and see if you can see tiny little bugs moving around, or eggs attached to the feather shafts. If it's parasites you will need to treat that. There are a lot of ways to treat, I use Sevin dust, but everyone swears by something different. If you do a search on 'mites' or 'lice' you'll find many threads with lots of advice.