I am really new to having backyard chickens. I have three hens(approximately 17 weeks old). One of them (Silver Lace) is experiencing major feather loss on her backside near her tail. There is a rather large bald spot. They have good light, always have food, clean water and ventilation... help! Any ideas or suggestions? Any advice would be greatly appreciated. I have a couple pictures I can send if that helps. Thank you in advance.