I have a concrete birdbath that is over 20 years old and has developed cracks too. A couple months ago I went to the hardware store and the manager recommended a caulk that is used in fish tanks. It is aquarium sealant, 100% silicone, and safe for fish. Haven't gotten around to repairing the bird bath yet, so I can't tell you how well it works!

Maybe I will do that in the next couple days, as it is supposed to be nice weather and it has to dry and cure for 48 hours.

Once you waterproof the birdbath, can you clean it well?

After researching, the advice to use aquarium sealant seems safest. It doesn't contain any mold inhibitors that other silicon sealants have, and is very strong.

Funny you should bump this. I finally got around to repairing that bird bath about a month ago! The aquarium sealant worked GREAT. I applied about 1/4 inch bead of sealant and smoothed it flat with my fingers, which was probably too much. I don't think very much at all would have been necessary to prevent leaking.

The only weird thing is that the sealant is slightly shinier and darker than the concrete, but you don't notice that when it's full of water.

Also, concrete bird baths are difficult to clean anyway. I use a stiff scrub brush. The sealant hasn't made any noticeable difference.