I've bathed all 7 of our adult chickens a few times, for various 4-H shows and events. They all got used to it after the initial panic when they were first put into the tub. I use 3 large rubbermaid plastic tubs: one with water, liquid ivory soap, and 20-mule Borax for washing; the next is the first rinse containing plain water and a capful of Clorox bleach (for parasite control); and the last is a final rinse with water, vinegar, a small amount of Glycerin, and a few squirts of Mrs. Stewart's Liquid Bluing, which helps to brighten up the coloring on light colored birds and adds a nice shine to the feathers. The birds will look their best a few days after bathing, once their natural oils have been worked back into the feathers.
I usually pour water over the bird's head quickly during each stage and they are okay with it. Just do it quickly enough that the bird doesn't attempt to breath at the same time, so they don't inadvertantly suck in water instead of air. I haven't had a problem doing this, and none of my birds has ever gotten sick afterwards. For extreme dirt issues, I would use my fingers or a wash cloth to scrub lightly in the direction of the feathers and then repeat the process. I've often had to scrub away poo buildup under the vent, which is easy to do while the bird is immersed in the warm wash tub. As someone pointed out, a soft nail or vegetable brush is a great tool for scrubbing the scales and claws clean.
The hardest part with this "assembly line" method is keeping the water warm enough for the birds at the end. The warm water tends to help them relax, as it's obviously more comfortable than bathing in cold water. The birds also tend to relax easier if they are well-supported by a hand under them, to make them feel safe. They should not be allowed to stand, however, or they will attempt to fly out... leading to much flapping and a water-sogged mess! Once they succumb to the inevitable, they often doze off so watch that the head doesn't fall forward into the water, to prevent accidental drowning.
If you only have one or a few birds to wash, a sink will work for each stage... changing the water and additives as needed with the advantage that the water will always be clean when you start. It's important to keep the birds warm and out of any drafts while they dry. Blow-drying is the best option, but you should still keep them indoors if possible, until they completely fluff out and dry underneath. (I usually keep mine in the house overnight after bath day.) I also find it easiest to trim nails and/or beaks after bathing, since they are softer then, as well as cleaner; making it easier to see the blood line/quick. The birds also tend to be more relaxed and compliant. I often complete this step while the birds are still wrapped in a towel, in fact.
Always keep some Blood Stop powder on hand, though, because it's super easy to misjudge and birds bleed A LOT when you hit the quick! I tend to wash my largest rooster first, because I ALWAYS nick a quick on the first one I trim, no matter how careful I try to be... I figure he's the strongest so he can stand to lose a little blood, rather than a smaller or weaker bird. He's come through fine each time, I might add. Btw, I adopted my particular method after reading a number of online articles and then combining the parts I liked best about each, including the link posted above.
