I have a year old black mix hen who about two weeks ago decided to go into a nesting box in her coop and not leave. I thought she was just broody at first but it's going on 3 weeks now and I noticed she is losing all her hair on her head. Her head is nearly bald but the rest of her feathers look OK. I have 3 other Welsummer hens who look and seem just fine. Any thoughts?