Rinse out the bird bath with the garden hose to remove bird droppings, debris and anything else that's fallen into the bird bath. Tip the bird bath over to empty the water out, then stand it upright again.
Pour ½ cup bleach into the bird bath. Use the hose to fill up the bird bath. Cover the bird bath with a large plastic drop cloth to ensure birds and wildlife can't get to the bleach water. Bleach is toxic, and can be fatal to these creatures.
Allow the bleach solution to remain undisturbed in the bird bath for 20 to 30 minutes. Remove the drop cloth and put on rubber gloves, then scrub the bird bath with a scrub brush. Wash the entire surface to remove grime and mildew.
Pour the bleach liquid out of the bird bath. Rinse the bath for a few minutes to wash out all bleach residue. Saturate the ground beneath the bird bath to rinse and water down the bleach solution emptied onto the ground.
Allow the bird bath to air dry completely. If the top is removable, prop it up on its side to aid the drying process.