Dry it out, clean the area and smear a thin layer of aquarium sealant over that spot.

Sealing leaking things from the outside seldom works as well as sealing them from the inside.

back when i was a kid on the farm over 60 years ago we would soften bees wax or parrafin canning wax and make seals aroun windows or seams ect. we would just soften it and press it in place and it was water proof for awhile.
what i suggest you might try is to make sure the bird bath is dry either leaving it in the sun for awhile or use a hair dryer to dry it. and then rub in some softened wax over the area that is leaking.

Thank you. Sealing with bee's wax sounds like a great idea but the aquarium sealant sounds easier. I'll try that first. I was thinking of painting the whole outside with something like Thompsons water sealer for concrete. But sealing on the inside makes more sense.

I have a concrete bird bath that has some cracks and it partially leaks. I bought the aquarium sealant, but haven't gotten around to fixing it yet (have 3 or 4 other bird baths).

This "water glass" sounds interesting. I looked it up on Wikipedia and it says "Concrete treated with a sodium silicate solution helps to significantly reduce porosity in most masonry products such as concrete, stucco, plasters. A chemical reaction occurs with the excess Ca(OH)2 (portlandite) present in the concrete that permanently binds the silicates with the surface making them far more wearable and water repellent."

Does it make the surface smooth and glassy? The surfaces on a bird bath need to be gritty and have some traction or they can be difficult for the birds to use.

Thanks for all the help. I painted to outside 3X with DryLock concrete water sealer but it still leaks. Can't figure out why. I like the idea of waterglass or Drylock inside but I'm afraid it will be harmful to the birds. I think the leak is bigger than I originally thought.I think it's too big for bee's wax or aquarium sealant. What about mixing up some quick dry cement, addiing some sand, and painting it all around the inside?

terrene:

The fact that Waterglass was used to preserve eggs seems to argue against it being harmful to birds.
It does leave a surface with a slight "sheen" to it when used on dental plaster and stone. In the event you think it might be too slippery for the birds why not sprinkle some sand on the wet waterglass, should work just like non-skid paint.