Cliff Swallow. While these small ariel insectivores do nest on steep cliffs (as their name implies), they often build their nests on manmade structures, such as this outhouse in Yellowstone National Park. Often you’ll see large groups of Cliff Swallow nests (sometimes 1,000 or more nests in a colony) owing to their highly social behavior. They build their nests out of mud.
Cliff Swallows are quite similar in appearance to Barn Swallows, but there are some obvious differences that make field identification less challenging. Cliff swallows have a white forehead and a short square tail. Barn Swallows have orange foreheads and long, deeply forked tails making them easy to differentiate in flight. Additionally, Cliff Swallows are pale underwing where as Barn Swallows are more buffy orange from below.