There was a discussion on a board I’m on concerning hens whose butts are bare. While my post on molting or pecking problems may address lack of feathers in part, bare butts may also be due to laying activity, especially in birds that lay well. This article has some good information and pictures: http://www.hencam.com/henblog/2010/10/bare-butts/
Good layers can end up with feather loss due to putting protein into the eggs rather than into keratin-rich feathers. If protein deficiency is severe enough, they may eat feathers. Always feed a balanced layer ration (16-18% protein). You can supplement protein with dry cat chow (a handful per day) or with high-protein chick starter (NOT a good idea for pullets, as this can cause them to go into lay too soon).
Another possibility is external parasites, such as northern fowl mites or poultry lice. You can manage these issues by adding wood ash or sulfur to dust baths or by dusting the birds and spraying their coop with external parasite pesticides, such as pyrethrin or permethrin. There may be a withdrawal period before you can harvest and eat the eggs. Follow instructions on any parasite products to obtain optimal results and keep you and your birds safe.
Concerning feather loss, it is important to ask, what is the underlying cause? Feather loss pattern can help you determine what is going on: aggression, parasite, nutritional deficiency, reproductive behavior, something else altogether!
Home to Roost 
Posted by Richard McGinnis on February 28, 2012 at 12:11 pm
All of my turkens have naked butts.
Posted by Sarah on May 28, 2012 at 7:43 pm
I bought three RI Red/ Hamshire mixes almost 8 months ago from a farm and at first two only had the bald spot on their bottoms now all three have balding ant my White Rock and Buff Orpingtons are thinning around the vent as well. All are still laying normaly. It’s just so sad to see.
Posted by Ron on July 29, 2020 at 1:18 pm
Most of my eight chickens have feather loss on their butts. Could it be the one bird with no feather loss causing this?
Posted by chickenwoman on August 15, 2020 at 10:08 pm
Hi, Ron – it could be one bird picking on the others. You might check for external parasites, such as lice, too.
Posted by Layla on November 10, 2022 at 5:50 pm
We’ve had a problem with poultry lice in the past, it’s a lot better now but one of my hens has a swollen bum, it’s red and mostly swollen between her legs. Could this be the lice or something different?
Posted by chickenwoman on November 11, 2022 at 10:15 am
Hi, Layla. If they’re clear of parasites, the redness could be due to the skin being exposed. If the abdomen is swollen, it could be ascites (water belly). I’ve also seen a few cases where a bird has had a (likely benign) turmor under the vent.