Bald Spot on Butt

[coach723]

I see the picture now, that very much looks like feather picking to me. The feathers also look very dirty, so she's probably getting dirty sitting in droppings overnight since she's not roosting. If any of your birds are over 18 months old, then it's likely they have molted, or if they are near that age then they are due. Some birds molt so gradually that it can pass without notice. Sometimes the only evidence, unless you look at them closely, is feathers all over the coop/run. Occasionally a bird might miss molt, or only do a partial molt, but they generally will molt once a year once they reach that age. When it's not very noticable and it's gradual it's called a soft molt. When large amounts of feathers are lost at once, leaving bare spots, or they have so many pin feathers that they look like a porcupine, that's a hard molt. Most molts happen late summer, fall, early winter, but the birds do it when their body says to, so it really can happen any time. Stress and illness can sometimes trigger a molt.
https://www.wideopenpets.com/everything-you-need-to-know-about-molting-chickens/
I have had dominant hens in the absence of a roo kind of take over his role, including mounting behavior. I had one that just became completely obnoxious, I was without a roo for about 6 months. Once I had another mature roo in residence she settled down and was much better.

 

[dalguiyeowang]

I see the picture now, that very much looks like feather picking to me. The feathers also look very dirty, so she's probably getting dirty sitting in droppings overnight since she's not roosting. If any of your birds are over 18 months old, then it's likely they have molted, or if they are near that age then they are due. Some birds molt so gradually that it can pass without notice. Sometimes the only evidence, unless you look at them closely, is feathers all over the coop/run. Occasionally a bird might miss molt, or only do a partial molt, but they generally will molt once a year once they reach that age. When it's not very noticable and it's gradual it's called a soft molt. When large amounts of feathers are lost at once, leaving bare spots, or they have so many pin feathers that they look like a porcupine, that's a hard molt. Most molts happen late summer, fall, early winter, but the birds do it when their body says to, so it really can happen any time. Stress and illness can sometimes trigger a molt.
https://www.wideopenpets.com/everything-you-need-to-know-about-molting-chickens/
I have had dominant hens in the absence of a roo kind of take over his role, including mounting behavior. I had one that just became completely obnoxious, I was without a roo for about 6 months. Once I had another mature roo in residence she settled down and was much better.

This is a dumb question, but do you think she's the one picking them or one of her sisters? She and her fellow cochin enjoy sleeping under my other girls (the ones who roost), so I know they end up getting pooped on often. I'll try your suggestion about putting them up on the roosting bar at night so they are not sleeping in any poop/getting pooped on from the others.
Of course now that I have posted her photo and her issue, she is growing feathers back, but I'd rather find out the cause of her bald patch.
I've laid down wee wee pads to accommodate her nest time (they have taken a straw bale and carved out their own nest--they used to use the milk crates packed down with fresh nesting pads, but t hey have claimed the straw bale) as I don't want the straw to irritate her already inflamed skin. I'm wondering if I need to remove their homemade nest and encourage their old arrangements.
Thank you so much for your help with this!

 

[coach723]

I think it's more likely that other(s) are doing it to her. I personally would try to figure out why they don't want to roost, and remedy that. When things become habitual to them, and comfortable, then they can be reluctant to change, so it may take some time. If you only have one roost bar, then if space permits, adding another one may help. Some birds like to snuggle close, and others demand space, so having plenty of room for them to choose usually makes things go better. No one wants to be close to a grumpy hen that wants space. I usually don't have birds avoiding roosting unless they have a leg/foot issue like bumblefoot, are sick, are broody, or sometimes if they are having a miserable molt and don't want to be near anyone.

 

[dalguiyeowang]

I think it's more likely that other(s) are doing it to her. I personally would try to figure out why they don't want to roost, and remedy that. When things become habitual to them, and comfortable, then they can be reluctant to change, so it may take some time. If you only have one roost bar, then if space permits, adding another one may help. Some birds like to snuggle close, and others demand space, so having plenty of room for them to choose usually makes things go better. No one wants to be close to a grumpy hen that wants space. I usually don't have birds avoiding roosting unless they have a leg/foot issue like bumblefoot, are sick, are broody, or sometimes if they are having a miserable molt and don't want to be near anyone.

I have bars on the opposite side of the coop--when I had my older hens were alive, they would take one side and the younger girls would take the other. I might leave Dorothy alone/not handle her too much because she is definitely in a molt. For now she's going to get extra protein and extra space.
Thank you again for your advice and for your help. I'll see if I can get her sister to practice roosting in the meantime.