How do cliff swallows build their mud pellet nests?

From the Cornell Lab of Ornithology, watch as some cliff swallows gather small chunks of mud from the water’s edge, and then, against a cliffside or man-made wall, stack the mud pellets around themselves like 1,000 drippy bricks. The sand, silt, and clay mud that they choose keeps the nest from crumbling as it dries. The birds might also line the inside of the nest with grass and feathers.

In nesting colonies of up to 3,700 nests, Cliff Swallows can be found in North America during the spring and summer. From AllAboutBirds.org:

One easy way to find Cliff Swallows is to look for their gourd-shaped mud nests clustered under horizontal overhangs—many a highway overpass is swarming with Cliff Swallows in summer. To find these birds while they’re out foraging, head to a lake, river, or wetland and seek out foraging flocks of swallows. Scan the swallows carefully, focusing on finding a square-tailed bird with a pale, pumpkin-colored rump and dark upperparts. Scan the upper levels of a foraging flock, as Cliff Swallows often forage higher than other species.

Listen to their calls at BirdNote.org, or read more at AllAboutBirds.org.

Related watching: more videos with nests, including the Weaver Birds intricate nests and the Burrowing Owl’s Cozy Underground Home.