Birds
Cliff Swallow
Petrochelidon pyrrhonota · also called cliff swallows
Beneath a highway bridge, hundreds of cliff swallows come and go from an equally large number of enclosed, gourd-shaped mud nests, each one built beakful by beakful into a colony packed tightly enough to rival any apartment block.
Overview
The cliff swallow, Petrochelidon pyrrhonota, is a colonial songbird native to open country across most of North America, nesting on cliff faces, bridges, culverts, and buildings and wintering in South America. Its uniquely enclosed, gourd-shaped mud nest, tendency to form massive breeding colonies, and successful adaptation to human-built structures make it one of the more architecturally distinctive and socially complex swallows on the continent.
A nest built one mouthful at a time
Where most swallows settle for an open mud cup, the cliff swallow commits to a fully enclosed structure, a project that takes hundreds of individual mud-carrying trips to complete before a single egg is ever laid.
Status
Least Concern — widespread and common across suitable open habitat throughout most of North America, aided by the widespread availability of bridges and other artificial nest sites.
Cliff Swallow: common questions
How does the cliff swallow build its distinctive nest?
This species constructs an enclosed, gourd-shaped nest entirely from mud pellets, with individual birds making hundreds of separate trips to carry small beakfuls of mud back to the nest site, gradually building up a fully enclosed structure quite different from the open mud cup nests built by relatives such as the barn swallow.
Why does the cliff swallow nest in such large colonies?
This species nests colonially, often forming large, densely packed colonies with hundreds or even thousands of nests built close together on a single cliff face, bridge, or culvert, a social nesting strategy that may offer benefits such as shared predator vigilance and more efficient foraging information exchange between neighbours.
Why are cliff swallows often found under bridges?
This species has adapted well to human structures, frequently building its mud nests under bridges, on culverts, and beneath building eaves in addition to natural cliff faces, since these artificial structures offer the same vertical surfaces and overhead protection the species originally relied on cliffs to provide.
How can you identify a cliff swallow in flight?
Look for a pale, buffy-orange rump patch, a distinctive field mark clearly visible as the bird flies, along with the species' tendency to forage and nest in large groups rather than alone, both useful clues for separating it from other swallow species sharing similar open habitat.