I have 6 Australorps that came together in August. I also have 1 Rhode Island Red. 3 of my Australorps (6 mo. old) have bald spots about the size of a quarter on their backs where their tail feather starts. The skin isn't broken or red, just missing feathers. Any ideas? How do I treat it and what can I do to stop it? Also, in Spokane, Wa where we will have below freezing weather this winter, do my girls need to dust bathe? How can I provide the materials for that?