Run a bin full of warm, soapy water and give her a good bath. I'm not sure what it is - vent paste?? - but it would help to have a picture of her cleaned off butt too to see if there's any swollen tissue or broken skin.
Subtropical desert (Köppen: BWh)
Elevation: 1090 ft Annual rainfall: 7"
It looks like pasty butt syndrome or maybe/possibly egg peritonitis (broken egg inside the bird). I would add a few tablespoons of applecider vinegar in a gallon of their water and wash the birds butt with warm, soapy water, like mentioned above. Remember, chickens are resilient birds if allowed to be chickens and this will probably clear up on its own after a bit of care.
I would say it's vent gleet. Try plain yoghurt; both on her bum and feeding it to her. Vent gleet is essentially a chicken yeast infection. If there's broken skin, you may want to skip the yoghurt since it's acidic, and use an acidophilus capsule, broken open mixed with water. However, if there's a lot of broken skin, it's probably not vent gleet. Is she standing on one leg and looking very ill? That would point to the earlier suggestion of a broken egg inside her. The nice thing is, the yoghurt won't hurt even if it isn't gleet, and I agree that some ACV could do some good as well.
Emily Wilson www.blarnyardgarden.blogspot.com
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