It looks similar to some of my birds. My buff orpington looks like that from the rear, and a lot of my birds have areas that are bald. One of my birds started picking and then taught the others (they just watched and started, also). I've seen it posted many times that this is evidence of a protein deficiency (picking and eating feathers), and we upped the protein in their diet with no change in behavior. We also gave them things to play with and perch on with no change. I bought some pinless peepers and we put those on all but the EE's as one does not need it, and they won't stay in place on the other one. After doing this, there has been a huge reduction in picking, and some of the birds are regrowing a lot of their feathers. The BO is wearing an apron, as well. Her bottom still looks just like your bird's, so I suspect that we have a bird or two who have learned to pick at least some feathers despite the peepers. I don't know what to do to totally break this cycle, but maybe there are a few hints in here to give you a few things to think about. Good luck with your birds.