Their nests look like they are constructed from spitwads. Both barn and cliff swallow use mud pellets to build nests that they attach to buildings, bridges and other structures. Both species winter in South America and return to Colorado each spring to nest. They often use the previous year’s nesting sites, since their mud nests are sturdy and resilient.
Barn swallows and cliff swallows have similar habitat needs, but very different habits. Both need open habitat to forage for flying insects. The both require a supply of suitable mud, a source of fresh drinking water, and structures where they can attach their nests. However, cliff swallows are gregarious and nest in huge colonies, while barn swallows are mostly solitary and rarely nest near each other.
Barn swallows are 5 ¾ to 7 ¾ inches long and are the only swallows with a long, deeply forked tail. They have steely iridescent blue uppers with a rust-colored forehead, throat, breast, and abdomen. Cliff swallows are slightly smaller, five to six inches in length, and have square tails. Their uppers are steely blue like the barn swallows, but the cliff swallow has a white forehead, buff colored rump, light belly, rusty face and pale gray nape.
Cliff swallows, as their name implies, originally nested on cliff and canyon walls or along the banks of large streams and rivers. Manmade structures like buildings, bridges, irrigation canals, and reservoirs have increased the amount of nesting habitat available for these birds. Cliff swallows build gourd-shaped nests with an entrance tunnel that opens downward. They use only mud pellets to build the nest and then line it with small amounts of grass, hair and feathers. The first arrivals of the colony usually build at the highest point possible and later arrivals attach nests just below those first nests – forming a dense cluster. Most cliff swallow colonies have hundreds of nests. Both males and females work to construct the nest, which typically requires over a thousand pellets. Since each pellet represents a trip from the nest site to the mud source and back, it can take well over a week to build a nest.
Barn swallows nests were historically built on sheltered cliffs or in caves and crevices. Now, they are usually built on beams or walls of buildings in urban, suburban, or agricultural areas. These birds like sheltered places for nests but also like to perch on power lines. Barn swallow nests are an open cup-shape made of mud pellets mixed with grass. Barn swallow nests are thickly layered with hair, grass, and feathers.