You shouldn't put anything in the bird bath that could affect birds, who bathe AND drink the water. Having clean feathers is imperative to their well-being. The water in birds baths should be clean.

If you do a quick rinse of the vessel with your hose and then refill it, say...every 2 or 3 days, you'll never have adult mosquitoes emerging from your bird bath again. It takes about a week for a newly deposited egg to emerge as an adult.

If the water has been in their long enough to hatch crops of mosquitoes, it's dirty and unsafe for the birds.

If you do keep the water clean and are still worried about mosquitoes, there's a nifty little device you can use in your bird baths that keeps the water vibrated (mosquitoes HAVE to have calm water). Mosquitoes hate it and birds love, love it. They're called water wigglers.

Here is a link that might be useful: click here to see

We use little donuts made with BT, a biological agent that doesn't harm the birds.

Bt-israelensis (Bt-i)

The BTi is safe for birds and application is recommended for bird baths, but I could never understand why. The bowls (if used by birds) are dirty within a day or two at the most and need to be rinsed, refilled with fresh water - long before mosquito larvae would have time to hatch.

Just an enthusiastic bathing robin or two can displace about half the water in a bath, leaving the rest pretty murkey for the drinking birds :)

I agree! I've never understood why people would use Bt, either. My baths are sprayed out and refilled daily, just because they get so yucky from those dirty little birds.

It takes something like 2 weeks for the mosquito larva to hatch from the eggs and even longer then that to become the adult mosquito so there is no good reason why a kind of almost tended bird bath should be producing any mosquitoes. If the bird bath is properly tended, ie flushed at least every other day there is simply no way any mosquito would develop.
If a bird bath is not flushed at least every other day there is a potential that diseases could be spread among birds using that bath, No one that has a bird bath should allow it to go more than 2 days without flushing it. There is no good reason to add anythjing but water to any bird bath.