I have a few chickens that are missing their neck feathers. Upon examination I see no insects or any other reasons for this to be happening. It has been going on for a month on some of my bantams but recently my standard Orpington Rooster has now lost a patch of his feathers. He is penned up with flock and the bantams are free range. Not all the bantams have this problem and only the one rooster in the pen is at a loss. I have in the past sprayed Adams Flea spray on them but have never seen any site of lice or mites. Any ideas?