In warm weather, a small, inexpensive “dunk” floating in the birdbath kills mosquitoes, but won’t harm birds or pets at all.
If you’re planning to put out a birdbath, remember that birds frighten easily. They’re small and (except for a few species) swim poorly. They shy away from water that’s more than 2½-3 inches deep. They also prefer a gradually sloping bottom, instead of a sudden dropoff to the birdbath’s full depth.