Do you have a rooster? Hens usually get a lot of wear and tear thanks to roosters and that's where they get it. If no roo have you seen the other hens pulling out her feathers?
You can buy a chicken apron which will cover the area and allow her to regrow her feathers without tempting the others.
I hope you enjoy being a BYC member. There are lots of friendly and very helpful folks here so not only is it overflowing with useful information it is also a great place to make friends and have some fun.
Including your general location on your profile will assist others. Location, climate, season etc can be important factors when members are responding to any questions you may have and vice versa.
If you would like to share Pictures and Stories of your flock, you have come to the right place. BYC’ers never tire of these and do not back away slowly or commence eye rolling when the photo album or home videos come out