I have used a product called Stick and Seal by Loctite. (Lowe's etc) on my bird bath cracks. It is a tube of rubbery stuff that seals out water, or in this case, seals in water. I know this is safe for birds because I use it to make things for my parrots. You might have to reapply it. It seems to shrink and dry out when left open to air.