I have one dirty/filthy/poopie butted chicken whom I've had to bathe on numerous occasions. I've brought her into the house and bathed her in the kitchen sink. The right side sink is filled with warm, soapy water, the right side sink is filled with warm clear water into which I've poured about a cup of white vinegar.
The chicken in question is very tolerant of the procedure and I'm sure feels so much better when she's done. The soaking off and cleaning of the nasty rear end can take a good 20 minutes, followed by good dunking in the rinse water. I then gently blot her with an old towel and blow dry her off, which can take over 30 minutes because I don't want to put her back outside into a Colorado winter if she has any dampness left.
I don't know what is wrong with this girl though. In a week she'll need another bath. I've taken her to the vet and tried everything suggested on this forum and yet, every time she poops it runs down her back end and sticks. She is shaped sort of differently than my other White Plymouth Rock and I suspect that she has some sort of physical deformity, although she contributes her share of eggs every week.
I'm trying to get up the resolve to butcher her, but I'm having a really hard time since she is so tame and agreeable in general.