Cliff swallows mine mud to make nests

By Jeff Mitton

Late in the day on the south side of Poncha Pass, I witnessed a spectacle of frantic avian industry: Cliff swallows were mining mud from a puddle.

They would approach from the north, wheel around and swoop down to approach from the south. They dropped into a mud flat beside the puddle, where the mud was just right — malleable, but neither soupy nor crumbly. Their feet sank into the mud, and they scooped mud into their beaks while fluttering their wings continuously. After a few seconds of working the mud in their beaks, perhaps forming it into pellets, they flew off.

I watched the mining for about an hour and wondered how many cliff swallows were in this flock. More birds arrived as others departed, but I could not determine whether it was a small group of birds returning quickly and frequently or a large group flying a considerable distance to their nests. Cliff swallows take advantage of nearby sources of mud, but they will fly for several miles if they must.

Cliff swallows were historically native to the mountainous portions of western North America, where cliffs were common. But in the last 100 to 150 years, construction projects, particularly masonry buildings, bridges and culverts, provided new nest sites that allowed swallows to expand their range to the east. They now occupy the Great Plains to the Smoky Mountains, and they reach the Atlantic in New England and Nova Scotia.A pair of cliff swallows builds their nest with 900 to 1,200 pellets of mud. The nests are placed on vertical surfaces directly beneath an overhang. This precarious placement greatly reduces the number of species that can reach the nest to eat the eggs or nestlings and probably reduces the exposure to rain, which could dissolve a nest of dried mud. Gourd-shaped nests have entrances near the bottom, and the entrance may be elongated to a tube up to 2 inches long. The inside of the nest is lined with dried grass.

Cliff swallows were historically native to the mountainous portions of western North America, where cliffs were common. But in the last 100 to 150 years, construction projects, particularly masonry buildings, bridges and culverts, provided new nest sites that allowed swallows to expand their range to the east. They now occupy the Great Plains to the Smoky Mountains, and they reach the Atlantic in New England and Nova Scotia.

Cliff swallows are the most social of the land birds in North America, with colonies ranging from several to several thousands nests. The largest colonies have been reported in Nebraska, where biologists have reported colonies of 3,700 and 6,000 nests.

Living in large, dense colonies has both benefits and detriments. Perhaps the most important of the benefits is finding food. When a bird has been unable to find food, it only needs to note which neighbors return with plenty of food. The hungry bird can then follow the successful birds on their next foray to a dense cloud of insects on the wing. Another benefit of many neighbors is that many eyes detect predators quickly, and a mob of swallows will immediately rise in defense.

Perhaps the greatest detriment to colony living is American swallow bugs, Oeciacus vicarius, which are to swallows as bedbugs are to humans. When nests share walls, as they do in dense colonies, swallow bugs can easily spread from nest to nest. Furthermore, they overwinter in nests when the swallows depart in the fall for Uruguay, Argentina and Brazil, so the populations of swallow bugs get larger as the years go by. After enduring infestations for several years, swallows abandon their nests and build a new colony.

Swallow bugs take blood meals from all in the nest. They are particularly hard on nestlings, reducing the number of nestlings that fledge. But a recent study has shown the blood meals also hurt adults. To investigate the impact of the swallow bugs, biologists fumigated some colonies and compared them to colonies with swallow bugs.

In colonies with swallow bugs, less than 2 percent of parents could fledge their nestlings and then raise a second family of chicks in the same summer. In fumigated colonies, 75 percent of parents were able to fledge two families in the same year. So a colony without swallow bugs produces many more chicks.

Jeff Mitton, mitton@colorado.edu, is a professor in the Department of Ecology and Evolutionary Biology at the University of Colorado. This column originally appeared in the Boulder Camera.

June 19, 2015