The inside is lined with hair, fine grass and bits of string. This intricate pouch can take up to two weeks to construct. Barn swallows and chimney swifts actually have benefited from human encroachment. Many of our structures provide them with ideal nesting habitat. Their nests are a variation of the cup nest.
Barn swallows build their nests of mud pellets.
Both parents deliver the pellets in their beaks to the nest site, which can be in barns, under bridges, on buildings and on cliffs. They construct a cone of mud and top it with a cup. Females then line the nests with softer materials, including feathers. Swallows commonly nest in colonies. Chimney swifts construct an even more unusual variation of the cup nest.