I had the same problem with a concrete birdbath that I found on the curb on trash day. If the cracks are visible, use some silicone caulk to seal them up---silicone is save and won't harm the birds at all. If the cracks aren't visible, or it just seems like the water is kind of seeping out, consider painting the outside of the birdbath with masonry paint. It will bind tight to the concrete and stop it from leaking.

I hope this helps! Good luck :-)

Is it safe to coat inside of mosaic birdbath with silicone caulk to keep it from leaking? will it harm the birds? If so then ...what else can I use.

Have a concrete bird bath that broke in half.Cleaned broken surface with steel brush. Coated both broken surfaces with bathroom tile quickset and pushed together.Let set a day.This is year 2 and like new-except for lighter color seam. Frank