On Mon, 30 May 1994, Marjorie W. Cleveland wrote:
 
> The following question came from another bulletin board, but the
> person asking for information received no response.
> He has a leaky concrete birdbath, and is looking for advice on
> how to seal the leakssafely, without poisoning the birds that
> drink from the birdbath.
> Anyone have helpful suggestions?
>
 
Get some aquarium silicone sealant from a pet or aquarium store.
It's designed to be in contact with water, without outgassing
toxins that hurt delicate creatures.  I don't know how well
it would do on concrete, but it would probably work OK if the
birdbath was drained and completely dry.