Birds need a dependable supply of fresh, clean water for drinking and bathing— few things are more attractive to them than a well-maintained birdbath. In fact, a birdbath in your yard may attract birds that don't eat seeds and wouldn't otherwise come to your feeders.
Traditional concrete birdbaths sold in garden shops make nice lawn ornaments, but aren't the best type for birds— they're often too deep and hard to clean and may crack during winter freezes.
Birdbaths that mimic rain puddles are best. These are shallow basins with a slight slope so birds can wade into the water.
Birds seem to prefer ground level baths. If you're concerned about cats, raise the bath two or three feet off the ground.
Keep the water level at 1/2" — 1" deep at the edges, sloping to a maximum of 2" in the middle.
Add some sand in the bottom to give the birds sure footing. If the bath is on the ground, arrange a few branches or stones in the water so that birds can stand on them and drink without getting wet (this is particularly important in winter).
Place your birdbath in the shade, near trees or shrubs if possible (but not so close that cats could pounce). The shade keeps the water from evaporating quickly and keeps it fresher. Birds wet from bathing don't fly well, so they're more vulnerable to predators. Shrubbery nearby gives them a safe place to hide while they preen and dry off.
One of the best ways to make your birdbath even more attractive is to provide dripping water. Many birds find the sight and sound of moving water irresistible. You can buy a commercial dripper or sprayer.
There are several ways to keep your birdbath ice-free in winter. Several manufacturers now offer birdbaths with built-in, thermostatically-controlled heaters. Alternatively you can purchase a separate immersion heater, available at most places bird feeders are sold. The latest models will turn off if the water in the bath dries up. Ideally, put your heater on a ground-fault interrupted circuit (available from any hardware or electrical supply store) to eliminate the chance of electric shock. A homemade solution is to put a light bulb in a flower pot under the water basin. The light bulb will provide more than enough heat to keep the water from freezing.
Never add antifreeze to the birdbath— it is poisonous to all animals, including birds. Some people use glycerin as a makeshift antifreeze in birdbaths, but we do not recommend it. Glycerin is a low-level toxin— if birds drink too much, it raises their blood sugar so much that they may die. Furthermore, when birds bathe in glycerin-spiked water, their feathers can become matted. Matted plumage is poor insulation, leaving birds susceptible to cold temperatures.
The secret to attracting lots of birds is to keep your birdbath full at all times. But remember to change the water every couple of days to keep it fresh and avoid the growth of algae. Most important, clean the birdbath regularly, to get rid of spoiled food particles and droppings, which may spread bird diseases.