It is good to refill the bird bath with clean water often. It heats up in hot weather and birds soil it as well. The birds always seem to know when ours has fresh clean water. They wait in line for a dip. :-).
07/10/2009
There are birdbaths with deeper dishes that would hold more water, but birds will not go into water if it is too deep. Since the dish is not very deep, mosquitoes will not have a chance to breed before the water evaporates, which is a plus. In hot weather, you often have to fill a small birdbath twice a day.