Keeping your birdbath mosquito free is not an impossible task. In fact, it is not even that difficult. It will take regular diligence on your part, but not a lot of time. Even more important, the acts you take to keep mosquitoes away from your birdbath also serve to get rid of potentially life-threatening bacteria and viruses that could endanger the same birds you are trying to attract.
Change the Water
The best way to eliminate any mosquito from your birdbath is to regularly change the water. Dump out the old water and add fresh water can be done with a garden hose or bucket. It is best if you can do this every day, but it needs to be done at least once a week. Changing the water eliminates any mosquito eggs that may have been laid there.
Clean the Birdbath
Cleaning the birdbath is another way to eliminate mosquitoes along with algae, fungi and bacteria in your birdbath. Birdbaths should be cleaned every one to two weeks. To clean the birdbath, empty the water from the birdbath. Use a dedicated scrub brush and scrub the surface with vinegar or diluted bleach (1 part bleach to 9 parts water). Do not use detergents as birds are very sensitive to them. Rinse the birdbath extremely well. Allow the bath to air dry in the sun before refilling it with water.
Aerate the Water
Adding an agitator to fountain to your birdbath not only eliminates mosquitoes, it helps attract birds. Birds prefer moving water to still water. Some birdbaths come with fountains installed in them. Solar and battery powered "water wigglers" are also available that can be added to birdbaths. These devices keep the water moving, which prevents mosquitoes from laying their eggs on the water.
Mosquito dunks are sold at most home and garden stores. These small cakes contain a type of bacteria that kills mosquito larvae. One cake works for about 30 days and is safe for people, pets, wildlife and fish. While mosquito dunks are effective and safe, they should not replace regular cleanings for your birdbath.