Cliff Swallows build their mud nests on cliff faces and other vertical surfaces in colonies containing hundreds, even thousands of other birds. They also build nests in the eaves of buildings. The pair works together, gathering pellets of wet mud in their beaks and laying them down one by one in rows until the structure almost reaches the overhang above. Swallows deposit mud pellets onto the nest with a shaking motion leaving behind well-formed nuggets that bond with the others and harden into a strong nest wall. Technically called an adherent cup, a finished nest contains over 1,000 mud pellets. Cliff Swallows tuck their nests away in corners and make sure that the nest opening is small so the eggs and nestlings are well protected from predators like hawks, owls, snakes, and mammals.