Cliff Swallow

The Cliff Swallow is one of the most colonial birds in North America. In the core of its range in the Western United States, colonies can contain thousands of nests, and those with hundreds are the norm. They build gourd-shaped nests of mud pellets on vertical surfaces, often so close together that they share a wall. Before European settlement these colonies were entirely on cliffs, but the species adapted quickly to use buildings and later bridges, with man-made structures supporting almost all colonies east of the Mississippi River.

Some of the more unique aspects of Cliff Swallow biology are associated with coloniality. Because nests are so close together, birds can easily engage in brood parasitism by laying eggs in other birds’ nests, or even moving them between nests by carrying them in their bills. Birds that engage in this behavior tend to produce more young since some are raised by other birds. Cliff Swallows are also host to a specialized parasite called the swallow bug (a relative of bedbugs) that only occurs in Cliff Swallow colonies. As bug populations grow over the course of a year their depredation can lower fledging success, and if numbers get too high a colony will eventually relocate if there is a suitable site nearby. The bugs are hardy parasites and can live three years in an abandoned colony.

Cliff Swallows need open areas to forage over, and this combined with the lack of cliffs meant they were historically largely absent from New Hampshire. This changed when agricultural expansion provided both buildings and open space, and the species increased in northern New England starting in the early 1800s. Fast forward two hundred years and the Cliff Swallow is now a Threatened species in New Hampshire. Numbers have declined by over 90% since 1970, and the number of colonies has dropped by 75% since the 1980s. As is the case with other swallows, biologists are unsure as to the causes of this decline, especially since populations on the Great Plains continue to do well.