Like all animals, birds need water to survive. With freezing temperatures many birds in urban areas are finding there regular bird baths and puddles frozen over. Do to this we see a large increase of birds in our backyard when a “freeze” hits.
This also means we need to change the water more frequently than daily because it will generally ice over during the day and make it difficult for the birds to get a drink.
An alternative is to get an immersion style water heater designed for bird baths which will keep the bird bath water from freezing. Try to pick one that turns off if the water dries up and make sure you connect it on a ground-fault interrupted circuit to eliminate the risk of electrical shock. (a ground-fault interrupted circuit is available at most hardware or electrical supply stores).
A homemade solution is to place a light bulb in a flower pot then put the bird bath on top of it. The warmth of the light bulb should be enough to keep the water from freezing.