It is important to clean your birdbath out every few days during summer. Giving it a good rinse with the hose might not be enough. Sometimes you need something stronger to clean away the algae or built up dirt. A solution of water and bleach might just do the trick.
Test this solution on a small part of your birdbath to make sure it will hold up to this mixture. If so, begin by cleaning off the birdbath with your hose. Then add water until your birdbath is almost full. To this add one to one and a half cups of bleach.
Make sure you cover this mixture with a plastic trash bag and secure tightly. You do not want any birds getting into this mixture. Leave and let soak for 15 minutes. Drain mixture in a safe place.
Rinse your birdbath for a good few minutes until the smell of bleach is gone. Refill with water and let the birds enjoy a day in the sunshine.