Minneapolis Star Tribune (subscription), MN – May 29, 2007
On The Wing Jim Williams
Last update: May 30, 2007 – 8:25 AM
A cloud of cliff swallows swirls 100 yards ahead of my van, flying above the road that bisects flat pasture in eastern South Dakota.
The birds appear on one side of the road, swoop up, perform dogfight maneuvers, then disappear on the other side of the road.
Most North American bird species have been affected by humans one way or another, some for the better, others for the worse. A species that has benefited significantly from the efforts of man — efforts never intended to involve birds at all — is the cliff swallow.
These birds, which were thought to have been limited to the southwestern United States up to 150 years ago, now breed almost everywhere in North America.
What did we do to help this species spread across the continent? We built bridges and culverts and buildings. Those structures meet the site requirements that cliff swallows have for their mud nests.
When they choose a nesting site, cliff swallows look for a horizontal overhang intersecting a vertical face. They require that the intersection form an angle close to 90 degrees. Because 90 degrees is one of our basic construction angles, many of our roadway structures suit the birds nicely.
Nest builders
Cliff swallows nest in gourdlike structures made of mud. As soon as the spring migration is over, groups of cliff swallows search for a nesting site. Once a site has been found, individual males choose where to build their nests.
The first nests begin at that critical right angle. Other nests are attached to those first nests, and eventually there may be hundreds of nests stuck together at one site. The nest entry begins as a hole, then evolves into a tunnel as as the nesting season progresses.
Cliff swallows require mud for construction, and will fly miles to find it. A pair of birds with good access to mud can deliver as many as 44 mud pellets to the construction site in 30 minutes. The average nest contains from 900 to 1,200 pellets, so sharing walls with a neighbor cuts construction time.
Cliff swallow nests, which are snug and often virtually predator-proof, sometimes are used by other bird species, including house wrens, house finches, house sparrows and Eastern bluebirds.
You’ve probably encountered cliff swallows before. Maybe you’ve driven across a river on a concrete bridge and found the air filled with swooping swallows. Or, perhaps you’ve passed a creekbed culvert in one of the prairie states and the birds have taken to the air as you rumble across it.
The next time you see them, stop to watch these builder birds.
Jim Williams, a lifelong birder, serves as a member of the U.S. National Wildlife Refuge Birding Initiative Committee. He also is a member of the American Birding Association, Ducks Unlimited, Pheasants Forever and Delta Waterfowl. He can be reached by e-mail at two-jays@att.net.
