I've never seen mosquito larva blow bubbles in the water, but I suppose it could happen. If there's a film of some sort on the water surface, then the larva could be causing the bubbles.
I am thinking of the birds first, though, when I say this. Any poison (I don't know what "dunk" is) that is designed to kill bugs could be dangerous to birds. We are talking about a bird bath, so if you aren't thinking about the birds first, maybe you should jsut get rid of the birdbath ... no?
"Give advice when asked. Give praise when appropriate. Give discipline when needed. Give respect always."