Birds that visit your yard appreciate the water you provide for drinking and bathing. But ceramic birdbaths sometimes crack in cold weather, and the tops of concrete birdbaths can be knocked off their pedestals and broken. You can repair them with the right kind of adhesive, and, once dry, the adhesive won't affect the birds' health. Glue toxicity matters if the glue is eaten or its fumes are breathed in. No studies link bird deaths with repaired birdbaths.
Clean the birdbath with a wire brush and dish detergent, especially the surfaces of the cracked parts to be joined. Let them dry. Using a wooden craft stick or similar item, mix epoxy glue on a clean piece of wood or cardboard and apply to one of the broken edges. Join the pieces. Wrap the bath with wire to hold it together while the glue hardens. Remove excess glue with a razor blade after 24 hours.
For stacked-glass birdbaths, use a clear waterproof marine adhesive. After cleaning and drying the broken edges, apply a thin layer of glue to both sides of the pieces to be glued. Wait two to five minutes for the glue to set and fit the pieces together. Tape or wire them to dry for 48 hours before refilling the bath with water.
For a terra cotta bath, use waterproof silicone caulk, which is non-toxic to human beings, as well. Clean the bath and its broken edges and let the pieces dry. Apply the caulk to one side of the broken piece and hold the pieces together with wire. Let the caulk dry 24 hours and remove the wire.
While using cement to repair a concrete birdbath seems logical, repairs made with cement tend to leak. Instead, use epoxy, which works as well on concrete as it does on glass and ceramic. Apply the epoxy to both of the broken edges. Wire together and let it dry 24 hours. Another adhesive, Bondo, formulated for car repairs, works, too.