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?