Warm welcome, Frank. Hope you enjoy this site.
I put AC vinegar in my chicken waterer along with a clove of garlic - they seem to prefer this water over the plain water in their larger metal waterer (the vinegar would react with the metal). In your birdbath, I'd just put a small amount in whenever you refill it. My ratio is one capful of vinegar to 1 gallon water. Supposedly helps with general health of the hens, so would likely be good for the wild birds as well. Here's a prior discussion:
http://garden.org/ideas/view/f...
