As cliff swallows return to the Colorado Plateau this spring, they set about building mud nests on cliffs – or, just as often, on manmade structures like bridge abutments or under wide eaves. That takes a lot of work—more than a thousand beakfuls of mud for a new nest.
Cliff swallows live communally, and they’ll sometimes fast-track the nest-building process by stealing mud from neighbors or laying an egg or two in a nearby nest.