This is my fifth winter here in a northern climate. It gets well below freezing for 3-4 months with a few 32 degree or above days sprinkled in and the ground is solid as a rock. When it gets to the the freezing stage I leave the birds inside to prevent any comb frost bite. I have not put in anything for a dust bath and after reading a few posts I am thinking maybe I should they would love it. Is sand the best thing or should I leave well enough alone and let them dig in the pine shavings.