You need to be careful with what gunk you are using in that birdbath, many
of these products can seriously harm the water quality. I've used Aquarium
sealer, it's non toxic and is made to work when submerged in water. You can
get it where home aquarium supplies are sold (pet stores). You smear it
on and work it down into the crack and then smooth it out to form a seal.
Just make sure everything is very DRY before you start your repair.

Patricia Hansen
Tacoma, Wa.
pmhansen at drizzle.com

"Use what talent you possess: the woods would be very silent if no birds
sang except those that sang best."
- Henry Van Dyke