Thanks in advance.
This is a long shot, but it was my experience.....
When the rats found our chicken's food, I raised the feeder. About that time I noticed the "runt" started getting a balding chest but didn't really think much about it since she was the smallest and would get picked on.
The rats still were able to get into the food so I hung the feeder even higher, but making sure that the chickens could still reach it. At that point the runt had lost most of her chest and neck feathers and I was freaking out. After researching it to death and coming up with nothing, I decided to watch their behavior for a few hours.
Turns out the runt was having to stretch to reach the food, and the galvanized feeder was rubbing her chest and neck. Multiply that by more than several times a day and we still have a small chicken with a bald chest and neck.
Hopefully it's something as simple as that.