It probably depends on where the leak is and how big it is... but I would try silicone. Some kinds are meant to fix aquariums, so it would hopefully work on this.

You could also try fiberglass patch.

I'm still looking for something to fix the wings on an angel and a couple gargoyles, maybe that fiberglass patch would work, other things give up in the heat, the wings just droop off.

I don't know if filling a crack is easier because theres no weight to bear, or if you have to find just the right thing to bind with the resin.

Hope you find something that works. If you do, please post it. And if you don't, I've seen some beautiful planters made out of leaky birdbaths.