Empty the dirty water and any other contents from the bird-bath bowl. Rinse out the bowl with water from the garden hose.
Fill the bowl three-quarters full with a mixture of one part chlorine bleach and 10 parts water.
Scrub the inside of the bird-bath bowl with a stiff-bristle scrub brush. Scrub all along the outer edge where the birds perch as well.
Discard the bleach mixture from the bowl. Rinse out the bowl well with water from the garden hose before refilling it.