Pour water out of the birdbath and allow the structure to dry.
Mix dry cement with water in a bucket to create a thick paste. Add water slowly, and add more cement if the mixture gets too loose. It should be thick enough that it will not drip from the trowel when held upside down.
Pick up a small amount of cement paste with the trowel. Spread the paste over cracks on the birdbath, pushing and scraping so the paste goes inside the crack. Spread a thin layer of cement paste over the filled crack to create a smooth surface that blends with the rest of the birdbath bowl.
Fill additional cracks on the inside and outside of the bowl and along the pedestal. Try to get the majority of the cement inside the cracks. Keep the fresh, exterior layer of cement as thin as possible to maintain the original shape of the birdbath.
Dip a paintbrush in water and brush around the patched areas to blend the cement and create an even finish.
Let the patched birdbath dry for about two hours. Cover it with a plastic sheet to finish curing overnight.