It is recommended, though, that a few sticks be laid across the birdbath to give the birds a place to stand while drinking so they can keep from getting wet. Birds should not bathe in the cold winter months.
Making a Winter Bird Bath
A special winter bird bath can be made by using an outdoor shop light and two weatherproof flower pots; avoid pots that tend to crack in the cold. Choose flower pots that are large enough to completely house the light without any of it hanging out of the pot and that have a large drainage hole in the middle of the pot. Remove the plug end of the light and run the wire down into the pot and through the bottom hole. Turn another pot over and run the wire into the bottom of the pot, putting the two pot bottoms together. Reattach the plug end of the shop light and place the bird bath bowl on top of the pots. The light bulb will keep the water warm enough that it won't freeze in cold temperatures.
Use a Heat Lamp to Warm a Bird Bath
Heat lamps can be very hazardous if they are not used properly so be sure to read all instructions carefully and never use the lamp where it will be exposed to the weather–use it only if your bird bath is under a roof such as on a porch or covered patio. Hang a heat lamp over the bird bath, about two feet away depending on how cold the weather is. This will provide heat to the bird bath and allow the birds to warm and keep the water from freezing.
Aquarium Heaters for Use in Low Temperatures
An inexpensive aquarium heater can be used inside the bird bath to keep the water warm enough so it doesn't freeze. This may not work in extreme cold and sub-zero weather, depending on how much the bird bath is exposed to the elements. Keeping the bird bath out of direct wind and placing it in a sunny, southerly exposed area will help to keep the water as warm as possible too.