It takes a little more effort to locate a solar powered bird bath heater, but it is possible to find models that use solar power to keep water from freezing in the winter. Some reports indicate that aiding wild birds through the winter with a steady supply of food and water can increase their chances of survival, so it's worth the effort to put out a solar heated bird bath if you live in a cold climate.
If you can't find a solar powered bird bath heater, you can add an to almost any bird bath. These heaters attach to your home's power and keep the water in the bird bath from freezing. There are dangerous drawbacks to using electric heat to warm your bird bath. First, you have to locate the bath close enough to your house so it can reach an electric outlet. Second, there will be an ugly and possibly unsafe cord running through your yard. Using electric power is of course more expensive than solar power, and having a cord may not be the best option around animals and your family.