The Mud Nests of the Cliff Swallow

June 14th, 2022

Have you ever wondered about those mud dwellings that birds build on the underside of bridges or on the sides of big buildings? Sometimes the number of birds flying around them seems enormous as you drive over that bridge. If they have rust or cinnamon colored rump patches, these are the industrious and beautiful Cliff Swallow, Petrochelidon pyrrhonata. Their face is also a rusty color with a white forehead, and their tails are squared off. Here on the mitigation lands in Little Lake Valley the viaduct highway has become a long bridge of homesteading opportunities for a few species of swallows, including the Cliff Swallow.

Their nests are made of mud dabs mixed with their saliva and then cemented onto the undersides of highway bridges, cliff ledges, building eaves, and even in large culverts. Cliff Swallows are one of our neotropical migrants that spend their winters and most of their lives in the outskirts of towns in South America but come here during spring to nest. They used to be more restricted to cliffs, canyons, and other natural nesting sites but they have adapted well to human infrastructure as well. They are the most colonial of all the swallows, forming colonies of up to 200 to 1000. (In Nevada a colony of over 3000 was found!) This year the Little Lake Valley colonies seem to have spread out and are in many sections of the viaduct where the cement supports hold up the highway.

Swallows of all species eat a tremendous number of flying insects, especially swarming ones. These include flies, bees, dragonflies, beetles, etc. According to Cornell All About Birds, both sexes of Cliff Swallow help build the nest, and the male may begin building before he attracts a mate. They gather mud in their bills along streambanks, lakesides, or puddles, usually near the colony but sometimes up to a few miles distant. They bring mud pellets back in their bills and mold them into place with a shaking motion. The finished nest is gourd shaped and contains 900–1,200 individual mud pellets. It measures about 8 inches long, 6 inches wide and 4.5 inches high, with walls 0.2–0.7 inches thick. The entrance, which is sometimes elongated into a tube, is about 1.7 inches high and 2 inches wide. The pair lines their nest with dried grass and continues patching it up with mud throughout the breeding season.

As I look up at the viaduct, I can see the distinct mud oval dollop-shaped “pellets” as they call them on the Cornell site, like in clay pot building. The nests are extremely strong and sometimes lasts through the winter elements. Sometimes old nests are used again or refreshed, not necessarily by the same pair that used it the previous year. European Starlings and House Sparrows, two introduced birds from Europe, compete for the “used nests” and often are in them before the Swallows are back from their winter homes. This competition has at times affected their numbers negatively in California, but worldwide the Cliff Swallow population has not decreased.

Some predators of Cliff Swallows are American Kestrel, Sharp-shinned Hawk, Peregrine Falcon, Acorn Woodpecker, and domestic cats.

The Cliff Swallow population along the viaduct is robust. They are flying with the Rough-winged Swallows, White-throated swifts, and the Purple Martins that also nest under the viaduct. These species are cavity nesters, and the cement structures have drainage holes that act as natural cavities.

It is a visual feast of fight acrobatics with great sounds and busy chatter to go with it!