BioEd Online

Roadside-nesting birds have evolved shorter, more maneuverable wings, which may help them make hasty retreats from oncoming vehicles, according to a study published March 18 in Current Biology.

The study’s authors discovered the evolutionary trend of shorter wings after noticing a decline in road-killed birds over the past three decades. They suggest that the two findings may provide evidence of roadway-related adaptation.

“I’m not saying that it’s all because of wing length,” says first author Charles Brown, a biologist at the University of Tulsa in Oklahoma. But, it supports the idea that these birds can adapt to disturbed environments, he says, and they’re probably not the only ones.

Charles Brown, along with his coauthor Mary Bomberger Brown, followed road-side populations of cliff swallows (Petrochelidon pyrrhonota) in western Nebraska for 30 years, mostly to study how they’ve adapted to living in colonies.

The migratory birds winter in South America but breed in North America in colonies of up to 12,000 adults. As their names suggests, cliff swallow colonies typically build their conical, mud-based nests onto the sides of cliffs, but have adapted to living under bridges and highway overpasses in recent decades.

As he checked the road-side colonies each year during the study, Charles Brown, an amateur taxidermist, also collected dead cliff swallows for skinning and stuffing—gathering 104 road-killed adults and 134 adults killed accidentally in nets used for the study. When he noticed a decline in road-kills, he compared the body measurements of both types of stuffed birds.

The Browns discovered that road-killed birds had longer wings, and the birds killed accidentally—which represented the general population—had a declining wing-length, which would improve their maneuverability.

 “There’s evidence that it allows the animals to turn more quickly—they can make a 90 degree turn more rapidly,” Charles Brown says. The extra agility of shorter-winged birds is useful for dodging traffic as the birds exit or enter their nesting sites, or take-off from the pavement, Charles Brown explains, which allows them to survive and produce more short-winged offspring.

Moreover, the decline in road kill can’t easily be explained by other factors, including changes in methods to find road-kill, traffic patterns, predators, diseases, or scavengers.

Bird researcher Johannes Erritzoe at The House of Bird Research in Christiansfeld, Denmark has also noticed a decline in road-killed birds around Denmark and suspects natural selection. Though he has not yet measured wing length, he says he plans to consider it now.

Though the Browns were able to discount many easy explanations for road-kill decline, they acknowledge that there are other possibilities that could explain the dearth of dead animals, such as the birds learning to avoid cars.

It’s hard to definitely prove that animals can adapt to living around roads, says behavioral ecologist Colleen St. Clair at the University of Alberta in Canada. But, “this is the best demonstrations that they do have that capacity.”