I needed to do the same thing last summer. I went to Home Depot and asked for help, and the salesman got me this concrete repair/patch paste. I did the repair, but the paste, quite yellowy tan, didn't match the birdbath (which was gray), and it was a pretty messy job.I think it did make the birdbath pretty watertight for awhile, but eventually it started to leak again.

I did try white caulking before that, and it didn't work very well at making the bath leakproof.

I think the extent and type of the cracking (jagged edges or long cracks, some of which you might not be able to see) would be tough to repair and of course that would make a difference in whether the birdbath can be made leakproof.

If you have a beautiful mosaic to repair you might want to talk to a mason, a hardware store employee who has done masonry work, or someone else who works with concrete.Who knows, maybe there is a way to make the clear caulking work - a mason would probably know. And that would be great, preserving your mosaic pattern. Good luck -

Donna