There are several things to consider when Selecting a Birdbath for your backyard. Birds, like many swimmers, want to ease into the water. A shallow birdbath with a sloped bottom is the best, and the water should be no deeper than 3 inches in the center. If you already own a deep birdbath, raise the bottom with rocks and pebbles. The rocks will make it harder to clean but it will be more appealing to the birds.Birds, like people, can lose their footing on a slick surface. Glazed birdbaths are pretty, but they will not attract as many birds as the classic cement birdbaths.