One of my five chickens (a GLW) is missing feathers on her belly/underside leading up to her vent. The skin looks sore and raw. She's happy and eating and acting normal, but it looks awful. It's been a hard, long first winter for the birds and they've spent a lot of time in the coop. That said the rest of the birds all look fine. Any ideas what this might be? Can I do anything to help her skin?