2 of my hens look like there molting, now they both have bare red bottoms. checked for mites,fleas etc.. found nothing. they look and act healthy laying fine. are they just the being picked on by the others? 8hens all together.
Could be picking, but once they start, they don't usually stop there. Is that scabbing that you can see in the picture? What do you have in their nest boxes?
Looks like an allergic reaction to something. Picking would be more on their backs not their tummys. Like wherever they are laying is something icky. You are not using redwood chips are you. They are not good at all!
What are they roosting on? Could she be rubbing her bottom on anything while roosting. I had some hens look like that and they were rubbing their bottoms raw at night. It stopped when we figured that out and renovated their sleeping quarters.
I'd be inclined to observe carefully for behavior and to treat the flock for mites/lice as a precaution. That looks sore!
I use pine shavings in there nest boxes and coop, that is all. I keep it clean and dry. they roost on a platform made from coated wire. there coop was Amish built. no changes in food.
I think it would be unusual for the others to be pecking her belly, but after watching them if you do find that pecking is the problem I highly recommend a product a called "Peck No More". Someone on this site suggested it to me and it worked wonders. I think some of the other thoughts about their roost area might be correct.