This is my hen with white stuff on her butt. The white stuff is on the outer edges of the feathers and appears nowhere else on any of the girls. I have never seen it on the roo. Sometimes I'll see this all day long, other days it seems to be there for a short time and then is gone until next time. I have treated with ivermectin for suspected scaly leg mite, then with permethrin spray for all other mites, and then with wazine for suspected gape worm--all with appropriate follow up treatment, cleaning of coop and space, etc. I see no hint of any pests anymore at all, but this white stuff persists. Several of them are molting, but this white stuff has been going on for some time now and doesn't seem to bother them at all. I'm hoping to hear that it's residue from egg laying or something else benign. Any thoughts?