Chickens are usually really good at preening themselves and don’t need our intervention to stay clean and fluffy. But sometimes some may need a little help, especially in their bum area.
The best way to wash a chicken is to give her a bath. Fill a tub with warm and soapy water, and put her in. Gently rub her skin and feathers until all the dirt dissolves, change the bath water if it gets too dirty. Then rinse her thoroughly and dry her with a towel. Finish drying her with a hair dryer, most chickens like this part of the spa treatment. It’s important that she is completely dry before returning her outside as she could catch a cold.
If you don't have time for a full spa treatment, you may trim the dirty butt feathers with rounded tip scissors. Be careful to not accidentally poke or cut her.
Here is a cute video of Janet talking about her bum bum feathers :)