Most of us put away our birdbaths during the winter months since concrete, ceramic and clay birdbaths can crack and break when the water freezes. However the birds still need water during winter for drinking and bathing.
My solution is a large plastic plant saucer filled with rocks. I take steaming hot water out each morning and pour it into the birdbath. Almost as soon as I go inside the birds are drinking and if the temperatures are more moderate they will bathe.