Excellent question! Water is one of the most important necessities for birds year-round. Most people understand the importance of water for drinking but many do not realize just how important bathing is for birds, especially in the winter. Because feathers are critical for flight and insulation, birds keep them well-maintained. A good part of a bird's day is spent just cleaning and grooming its feathers by bathing, scratching, and preening.
To keep open water available to birds, I like to use the Wild Birds Unlimited (WBU) durable, heated plastic birdbath which provides a reliable source of water when natural sources are frozen, even to temp. below -20° F. Mount it easily to deck rails or place it in a birdbath holder to allow visitors to drink, bathe and keep their feathers clean all winter. It features a built-in 150 watt, fully grounded heater that is thermostatically controlled to conserve energy. It also has a full 4 year warranty and it’s made in the U.S.A.
Another option is to place a birdbath heater in the bath you already have. WBU offers two heaters that turn on and off as needed like the heated birdbath. Our heaters are designed to be placed in concrete, plastic, resin or stone bird baths.
Or there is the birdbath de-icer. Its affordable foil construction is ideal for shallow birdbaths and stays on all the time at a low wattage to keep a circle of melted water in your bath. The 3-prong cord is fully grounded and has a one year limited warranty.