Your hen looks like she has new feathers growing in. You can't see this happening on other areas of the bird because the bird preens her feathers by breaking off the casing in her mouth, she removes the covering around the new feather and shakes off the broken casing. If you press and roll the casing together between your finger and thumb, the cover will fall off. It will look like a paper casing that you can blow away or brush off.
When my hens have an area they can't reach, on the head, behind the neck, and back at the vent for the older birds, they appreciate the attention to clean off the casings and open the new feathers. Watch your birds, look for them lifting a wing and grabbing a feather in her mouth. She is cleaning the feather, also known as preening. Put the bird on your lap, lift the wing and look through the feathers, there may be more feathers growing in throughout the bird's body and wings.
