The cliff swallow, or American cliff swallow (Petrochelidon pyrrhonota), is a member of the Hirundinidae family, which includes swallows and martins. The name derives from Ancient Greek words meaning “stone” and “swallow” as well as “flame-colored” and “-backed.”
Known for their social behavior, cliff swallows create large nesting colonies, sometimes exceeding 2,000 nests. They are often seen in large flocks during migration, foraging for flying insects or perched closely together on wires.
These birds construct gourd-shaped nests from mud with small entrances, typically under bridges or on cliffs. They engage in vocalizations to communicate warnings and food availability within their colonies. Adult cliff swallows measure about 13 cm (5.1 inches) and have dark brownish plumage, with a distinctive white forehead, red cheeks, and a square tail.
Males and females are identical in appearance, so they are differentiated through the palpation of the cloaca. During breeding, males have more pronounced cloacas due to swollen seminal vesicles, and females lose feathers from their lower breast to incubate their eggs.
Juvenile cliff swallows have paler plumage and distinctive white markings that fade with maturity. Traditionally nesting in cliffs along the western North American coast, they have adapted to urban environments, building nests on concrete structures as their range expands across Canada and the U.S.
In winter, cliff swallows migrate to southern Brazil, Uruguay, and parts of Argentina, although their behavior during this time is not well-studied. They are long-distance migrators, with eastern populations traveling through Florida and western populations through Mexico and Central America. The dynamics of their migration—whether they stay together in flocks or disperse at their destinations—remain unknown.
Cliff swallows primarily feed on flying insects, such as flies, bees, and moths, foraging at heights of 50 meters or more over fields and marshes. They are diurnal birds, returning to their nests at dusk, and tend to be less active in cold or rainy weather due to reduced prey availability. Upon arriving at nesting sites, they may forage up to 10 miles away to build body fat reserves for egg-laying.
Their breeding season begins when they return from wintering grounds and choose nesting sites, often observing from previous seasons. Younger pairs benefit from larger colonies as they gather valuable information, while older birds tend to nest earlier in smaller groups to avoid parasites. Nests are constructed from sticky clay and can last several years, with swallows often working in groups to collect mud.
Typically, a clutch consists of 3–4 nestlings, with some females engaging in brood parasitism, laying their eggs in other nests. These females often achieve greater reproductive success. However, nests are vulnerable to predation from house sparrows and snakes, particularly at the colony’s edges. House sparrows can significantly reduce the success rate of cliff swallow colonies by taking over nests.
