Good Hair Day Bad Hair Day

My girls are worth their weight in gold just for the entertainment value. We have an area under some enormous fir trees that is dry as a bone. Chickens never waste anything so they've made it their spa area.

They have excavations dug out deeper than they are tall. They jump in, start flinging dusty soil, and fling/squirm/fling/squirm until they feel they've gotten their groats worth of spa treatment.
Yet twenty minutes later they will be bellied up to the beverage bar sleek and glossy, all blue/green/purple iridescent. How do they get so clean so fast? There must be a room at the chicken spa I'm not privy to.