All my chicks have had this problem. I just bring them inside to the bathroom, run warm water in the sink, flip them onto their back so they calm down, then set their butts down in the warm water. You can rub their poopy feathers with a little soap too.
Of course, I've always done this in the summer, but this time of year you'll want to keep them warm.
Of course, I've always done this in the summer, but this time of year you'll want to keep them warm.