My hen has no feathers on her butt. Like where the egg comes out, not on her back. I don't have a rooster. Just 3 hens. I need to know why quickly so I can treat it. She doesn't seem to be bothered by it.
It could be either parasites, so I'd check for those first.
If you don't find any than it is probably the other chickens pecking at her. There are several reason why they'll do this - stress, boredom, or protein deficiency (they'll eat the feathers for that extra protein) - and once they get started it is hard to stop. Applying something like Blu-Kote will darken the skin, making the other chickens less likely to pick at it, and if that was the problem the feathers should eventually grow back.