My chickens all have missing feathers (bald spots) on their backs. At first we thought they were pecking each other but have never actually seen them do it. Now the spots are getting bigger. We read about mites and sprayed the coup and chickens with a pesticide that was safe for chickens. That was several weeks ago and we still see no improvement. Help! What is this and how do we fix it? I will add a pic to show what they look like.