Shorter-winged swallows evolve around highways

Crossing the road has gotten easier for cliff swallows. Over generations, the mortal threat of speeding cars may have shortened their wings.

Over the last 30 years, the number of cliff swallows killed along roads in southwestern Nebraska has plunged, and the birds’ average wing length has shrunk, researchers report March 18 in Current Biology.

The data are “jaw dropping,” says animal behaviorist Colleen Cassady St. Clair of the University of Alberta in Edmonton, who was not involved with the work. The results suggest that years of smacking into SUVs forced swallows to adapt to the road.

In the absence of roads, cliff swallows — sparrow-sized birds with orange rumps and white foreheads — tuck their nests under overhangs on cliff faces. But in the last few decades, many birds have traded ancestral homes for modern real estate — highway bridges and overpasses.

Cliff swallows can plaster thousands of cantaloupe-sized mud nests to the undersides of these structures, says study author Charles Brown of the University of Tulsa in Oklahoma. These colonies are less likely than cliff nests to be washed away in storms but come with a different risk: They perch near roads — and fast-moving traffic.

As graduate students in the 1980s, Brown and study coauthor Mary Bomberger Brown didn’t set out to study swallows’ adaptations to cars. They were interested in the birds’ social behavior. But because the team drove thousands of kilometers among colonies, they saw a lot of roadkill.

Every summer for the next 29 years, the team trekked to the colonies, counted nests and picked up dead birds. In total, the Browns gathered more than 2,000 swallows.

Starting in 1983, the researchers collected fewer birds killed by cars each year, until they found only four in 2012. And when Charles Brown measured preserved specimens’ wing lengths, he saw that, compared with the rest of the population, swallows that died on the road had wings that were a few millimeters longer.

A few millimeters — about the width of a Tic Tac — might seem like a small change, but for birds’ wings, “a little bit can make a big difference,” says evolutionary biologist Ronald Mumme of Allegheny College in Meadville, Pa.

Petite wings let birds take off quickly and maneuver deftly through the air. Like quail, which have short, rounded wings and can explode off the ground almost vertically, Brown says, swallows might be better served by short wings that help them whiz up and out of harm’s way.

He thinks the population’s shorter average wing lengths could help explain why roadkill numbers are going down. “It’s amazing what natural selection can do,” he says. 

The team ruled out other potential explanations, such as declining swallow populations or an increase in avian scavengers stealing carcasses. Still, Charles Brown says, factors other than wing length may also be involved. Cars may have killed off daredevil swallows, for example, leaving more cautious birds behind.