If you don't have a heated birdbath, you can purchase a small water heater designed for use in birdbaths that's invaluable in freezing temperatures. You can also buy pumps and drippers that add some motion to the water; this helps attract birds and discourages insects, such as mosquitoes, from laying eggs in the water. Finally, you can get a mister for your birdbath. A mister sprays a fine mist of water; some bird species, notably hummingbirds, can't seem to get enough of it.