Sometimes being more descriptive will help with answers. Is the abdomen swollen? Is there a syct or tumor--hard to tell from the picture. How old is the hen? Is she laying?
Ok a bit more description. Just now got out there and caught one of them ( they both the same in this regard ). It isn't a hard spot, feels the same as if I pushed in my own side just above the waist. She didn't pitch a fit at all when I pressed it so it appears to be zero pain response. They seem otherwise perfectly healthy. Eating, laying, Their vents are clear so not pasty butt. I guess that means it just a bald spot? Both of them have the exact same spot like that, maybe genetic thing if they are related?
The bald spots may be from feather picking. Make sure they have enough room, 16-20% protein in their feed, and some things in the coop to occupy their time and attention. Let them outside to range if possible. If there is any swelling under the vent, that could be internal laying/egg yolk peritoniits, tumor, or ascites.
This is Wolfram, heretofore the accused. I caught her picking. I thought she was low on the pecking order due to her size but I believe she may be running the show. Barred cochin bantam. Adorable hence I never suspected her. Think the mystery has been solved. Their new place is almost finished, 150 square foot for 4 birds, should eliminate this I hope.
*EDIT* I need to find a way to free range them a bit. Dogs and coyotes everywhere round here though.