Could be head lice. They are small, solitary and very fast. The hens can see them and try to peck them out, breaking the feathers in the process. We've had this with a few hens and sorted it with a small spray of Frontline. The top hen will also preen the cockerel's head feathers for him after a moult, removing the sheaths. She may not be very skilled at it and we have that problem here at the moment.