1) You can change the water often. You'll want to change the water at least once or twice a week. The typical mosquito goes from egg to adult in 7-10 days depending on the temperature/food available. By changing the water weekly you will prevent the cylce from completing. EFFECTIVENESS: 100%
2) Circulation. By having moving water the mosquito will not lay eggs/larvae can’t develop. This sounds good, but I have found mosquito larvae in moving water before (along the edges). EFFECTIVENESS:75-100%
3) Larvicide. A common larvicide (larvae killer) is available at most hardware stores. Look for the active ingredient of Bti (Bacillus thuringiensis isrealiensis) as it causes the larvae to die. It is VERY species specific and will not injure birds or any other creatures deciding to cavort in your birdbath. EFFECTIVENESS: 99.99999% (Almost perfect!!)
So don't let these pesty little insects turn you away from enjoying one of the most entertaining lawn and garden pieces you can have ever have in your garden...a beautiful bird bath!