It's no secret that roadside flying can be dangerous for birds. But new research suggests they may be getting a little help from a surprising source -- evolution.
After a 30-year study of cliff swallows, which often nest under bridges and overpasses, researchers in Nebraska found that the number of birds killed by vehicles has steadily declined since the 1980s. The team also found that the average wing length of the birds there has declined, concluding that these birds might have evolved shorter wingspans to avoid moving vehicles.
"Longer-winged swallows sitting on a road probably can't take off as quickly, or gain altitude as quickly, as shorter-winged birds, and thus the former are more likely to collide with an oncoming vehicle," study co-author Dr. Charles Brown, a professor at the University of Tulsa, said in a statement.
Analyzing the differences between the swallows that were killed and those that survived, Brown's team noted that birds hit by vehicles did indeed tend to have longer-than-average wingspans.
"Clearly, the birds that are being killed are different from the population at large; they do have longer wings," Brown told The Huffington Post. Thus, Brown explains, "we can see natural selection operating."
Although the research illustrates that wingspans have shrunk, evolution as a result of roadside casualties may not be the only cause.
"I don't think necessarily road mortality is the sole cause of [shortened wingspan]," Brown told NBC News. "I think there are other factors that may be leading to this, but the point is, I think that for whatever reasons, these animals can adapt very rapidly to these urban environments and they can avoid being killed."
In the study, Brown and his team noted that severe weather and changes in insect prey, as well as swallows learning to avoid collisions by observing other birds, may also have contributed to the shortened wingspans and the decrease in avian road kill.
Few road kill studies focus on birds, and the most recent statistics -- from 2011 -- estimate that 60 million birds are killed by cars in the U.S. each year.
John Hoogland, an evolutionary biologist with the University of Maryland's Center for Environmental Science, said the recent swallow study depicts a "beautiful trend that never could have been predicted."
"We humans, because we're changing the environment so much, are adding a new kind of natural selection to these animal populations," Hoogland told ScienceNow.
Brown's swallow study, a collaboration between the University of Tulsa and the University of Nebraska-Lincoln, was titled "Where has all the road kill gone?" and was published Monday in the journal Current Biology.