UNL study suggests swallows evolved shorter wingspans to avoid traffic

Mary Bomberger Brown announces cliff swallows have increasingly been able to evade death near neighboring roadways thanks to a shortening of their wings.
Mary Bomberger Brown announces cliff swallows have increasingly been able to evade death near neighboring roadways thanks to a shortening of their wings.

Cliff swallows living and building nests on highway overpasses and bridges have a better chance of avoiding cars and trucks than in the past because of a shorter wingspan – one that now helps them dodge those approaching vehicles, a new study co-authored by UNL ornithologist Mary Bomberger Brown suggests.

In an article published this month in Current Biology, Bomberger Brown and her co-author, Charles R. Brown of the University of Tulsa, found that their 30-year study of cliff swallows in southwestern Nebraska revealed a significant relationship between human activity and the evolution of the birds’ bodies.

In the past 30 years, the study suggests, cliff swallows have increasingly been able to evade death near neighboring roadways. In the past, oncoming vehicles often struck birds flying over roads or resting on highways. But the study found that such road kills have decreased over the past three decades, a fact that directly correlates to the physical shortening of the swallows’ wingspans.

“Evolution is an ongoing process,” said Bomberger Brown, a research assistant professor in the School of Natural Resources. “Things like roads and SUVs have all become part of nature or ‘the wild.’ They exert selection pressures in ways we don’t usually think about.”

The research, which is one of the longest known continuous evolutionary studies, focuses on a highly social bird species that frequently nests underneath highway bridges and overpasses. In the past 30 years, more and more swallows have begun nesting under bridges and overpasses, which places them into closer proximity with –- and in danger of -– cars.

“We started watching swallows at about the same time as (the swallows) discovered bridges,” she said. “It was like catching them at the discovery of the wheel.”
Initially focused on the social behaviors of the cliff swallow, the research also evolved into a broader study of the species, she said: “We began by banding the birds to track them. Every year, we’d go back to check on them. When we saw a road kill, we’d pull over the see if it was one of our birds.”

Even if it wasn’t, Brown and Bomberger Brown would collect data about the birds’ physical characteristics. They began to note that the birds found on the side of the road disproportionately had longer wings, a quality that made the birds less aerodynamic when compared with their shorter-winged counterparts.

“It’s like the difference between a U2 spy plane and a fighter jet,” Bomberger Brown said – shorter wings make for easier mobility, which allows birds to fly upward and pivot away from threats, like oncoming traffic.

The researchers were able to eliminate other potential causes for their published results, noting that bird populations increased even as road kills decreased. Also, studying the types of automobiles on the road -- especially the increase in SUVs, with their higher front-end surface area -- revealed that the decrease in mortality could not likely be causatively linked to the types of vehicles on the road.

The most likely cause, and statistically significant correlation, was wing lengths.

“It’s a lesson in how to collect data,” Bomberger Brown said. “We didn’t set out to study this question, but we collected as much data as possible. In the end, the data told us what we should notice.”

The results will affect the thinking of scientists interested in animal adaptation and human-nature interactions. The fact that cars created natural-selection pressure on the birds meant that birds with better-equipped physiques could adapt to their surroundings.

The added layer of complexity Bomberger Brown sees is that the cliff swallows now face increasing selection pressures to adapt to a changing climate, which scientists consistently link to human activity.

“We have already seen how weather extremes affect the birds,” she said. “Our data shows that our cars have had an effect on birds. Now we’ll see what the climate we’ve created will do.”

Is that where Bomberger Brown’s studies will go now?She said she was unsure.

“The birds will tell us what’s next,” she said.

Brown’s and Bomberger Brown’s findings in Current Biology can be found at http://www.cell.com/current-biology/fulltext/S0960-9822(13)00194-2.

--Michelle Hubele Rubin

More details at: http://go.unl.edu/pc7