Drain the water out of your bird bath.
Scrub algae away using a scrub brush. If any algae remains, make a paste out of baking soda and water, and scrub your bird bath with the mixture.
Hose down the inside of your bird bath to clear away the algae and baking soda.
Refill your bird bath with clear, fresh water.
Clean your bird bath and refill it with fresh water every few days to prevent future algae growth.