My hen has a bald patch about 2" x 3" on her back near where her tail feathers grow. It bothers her and she interrupts feeding to peck at it. All the new pinfeathers which were there are gone. I put on Pick no More on it, but she eats it off right away. I dusted her with DE and a pyrethium poultry dust thinking it might be mites, to no avail. Any ideas? How far back does a chicken apron cover? She is an adult Barred Rock.