Is an essential part of bird life, if you have a pond and it has shallow margins, that is best, but for those of us who use a bird bath, it becomes part of the birds territory, and so they expect to find fresh, clean, water all the time. This can be difficult during winter, and a twice a day job in the summer.
You can buy bird bath anti freeze. (Never use car anti freeze) but, if you can site your bath in a sheltered spot, or next to a heat source like the boiler flue, that may reduce freezing. Some people put an aquarium heater in the bird bath. In reality you can only do so much to beat nature, you may have to break the ice and replace the water twice a day in the winter as well as the summer.