Sometimes it's mites causing that baldness behind, there are threads on how to spot and treat mites.
Sometimes molting, which happens at this time of year, begins at the back end. They usually stop laying. I had a hen with a bare area all around her behind and it was kind of red. I examined her for mites, I didn't see any. Then saw the pin feathers coming in and thought ah, maybe just molting. Now it's all grown completely back. She had stopped laying for a month and now is laying again and has a new fluffy butt.
Sometimes molting, which happens at this time of year, begins at the back end. They usually stop laying. I had a hen with a bare area all around her behind and it was kind of red. I examined her for mites, I didn't see any. Then saw the pin feathers coming in and thought ah, maybe just molting. Now it's all grown completely back. She had stopped laying for a month and now is laying again and has a new fluffy butt.