Most of mine have the same bald bum, all but the head chicken. Apparently they pluck each others bums while on their roosting perches. It seems to get better in the winter, perhaps they are too busy trying to keep warm to pluck each other.
Good luck
I had 8 chickens ranging in age from 1-3years. I've lost 3 hens since February. my 9 month old silkie died suddenly in February, didn't notice any symptoms I hadn't been out to the coup in a few days as I was ill and my husband was caring for the hens. He just found her dead no signs of injury...