They usually nest as single pairs or in small groups but are not considered a colonial species. The Bank Swallow is a very small bird with brown upperparts and dingy white underparts. It is similar in appearance to the Rough-winged Swallow except for a broad, brown band around its neck and upper chest. This bird is known as the Sand Martin in Europe. Bank Swallows nest in holes that they excavate in the sides of river or lake banks or other steep cuts that provide sandy dirt in which to dig their nesting burrows.
Colonies can sometimes contain thousands of individuals.
I have a study of one colony in Europe that contained over 2,000 pairs. When I think of the Barn Swallow, I think of sheer grace and beauty. The Barn Swallow is probably the best known and studied species of swallow in the world. Its races enjoy nearly global distribution. The nest is an open cup of mud pellets and grasses lined with white feathers placed in a protected spot under a bridge, porch, or outbuilding of some type.