How old are the ladies?Thank you Wyorp Rock and Eggcessive! I've been watching and I've never seen the others go after her. She's one of the bosses in the flock. But, just now I examined her again and I am starting to see whitish clumps in crevices of skin below the vent. I consulted TeleVet and she suspects poultry lice. Some of the other hens have been pecking at their feathers lately and so I'll treat the whole flock. I hope this is going to solve the problem, I'll post an update.
They may be molting, but it would be good to look for lice and mites.
You have a black hen - can't tell if she's an Australorp, Jersey Giant or possible production bird. Skin color is dictated by breed. Australorp will have white skin, Jersey Giant will have yellow skin. A production breed (hybrid) could have white or yellow skin.
Naked Neck birds, while the featherless portions of their body does turn red due to exposure and sun, the rest of the skin underneath their feathers should be yellow.
Reddening of the skin can be from exposure, picking, etc. in laying hens so just keep watch on the area. NuStock, Hen Healer or similar type products can be applied to the skin if it looks like it's breaking down. Pine Tar can be used to help deter picking.