Have 2 SL Wyandottes and 2 Buff Orpingtons, all 2 1/2 years old, all laying hens. Both Wyandottes have lost the feathers to the rear of their backs where they are able to pick themselves (have pics); and the Orpingtons haven't! They are definitely not molting and I don't see any evidence of mites or lice. It's been this way about 6 months and they are laying fine. Help.