Strange feather loss in same area of all hens (see pics)

[MrsBachbach]

The hens aren't with the cockerels now. They haven't been for months. The feather loss is since the cockerels have been kept away from hens

I've seen threads about this type of feather loss and yours look the same. Search depluming mite. It lives in the feather shafts from what I read. I think one lady had to do several dips to kill it.