Looks like rooster damage from mating when they grab hold of the back of the neck. How old is the rooster and how many hens do you have with him? I would consider separating him in a pen for awhile to give her a break. Hens may also peck at the feathers, so you might want to coat her bare area with something like Nustock Cream, Pine Tar (both in the horse section of feed stores or online. BluKote can stain it purple blue, and may hide the bare spot. Even Vicks or something similar might keep them from pecking at it.