Every year sometime in November I empty out the birdbath and then turn it upside down until April. That eliminates the risk that it will crack when the water in it freezes during the winter.
This November I forgot to do that and last night the water in the birdbath froze.
This afternoon I brought the birdbath in and put it in a bathtub for about an hour. After the ice started to melt and separated from the sides of the dish, I was able to dump the block of ice and all the leaves stuck in it outside. Luckily, the birdbath doesn't seem to have suffered any damage.