Thanks for getting back to me.
The affected area is on the top-rear section of their backs. I have pictures but don't know how to add them.
I haven't seen any agressive pecking by the normally docile Orpingtons. The pattern also seems to show that the area is reachable by the Wyandottes...
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...