Cover the entire bird bath basin with your black plastic trash bag. This will keep the birds away from the bath so they do not drink or bathe in the chemically treated water, and the black color will absorb the sun's heat to heat the water and clean the bath more quickly. Pull the bag thoroughly over the basin and down the pedestal so it will not blow away in a breeze, or if you are cleaning a ground bath, weight the bag down along the edges to keep it in place.
At this point, leave the bird bath to soak for 10-15 minutes. This is a great time to refill or clean birdfeeders, make hummingbird nectar, check a birdhouse or tend to other birdy tasks in the backyard. If necessary, you can leave the bird bath to soak for far longer, but for thorough cleaning a minimum of 10 minutes is recommended. Very dirty bird baths may need longer soaking to be fully clean.