I felt so guilty this morning when, on opening the curtains, I saw a blackbird pecking at the ice on our bird bath. This friendly bird is a regular user of the bath and splashes around in it daily, usually emptying most of the water out in the process!
Before flying off to seek water elsewhere he appeared to give me such a 'look' as if to say "Well that's our friendship finished!" that I made de-icing the bath my very next job. I hope he forgives me.
You can always an a bird bath heater to prevent the water from freezing in winter. There are battery operated, solar and electric models. They are also not expensive!