The cliff swallow likes company when it is nesting: a single breeding colony may number several thousand pairs, all tightly packed on a favoured cliff face with the entrance tunnels to their nests spilling over one another. Males and females build their nests from hundreds of pellets of mud that are laid down one on top of other and left to dry hard in the sun. The pebbly structures that result are strong and long-lasting. Cliff swallows live in North America in the summer.