Cars and trucks thundering down the road in southwestern Nebraska stand a much lower chance today of smacking a cliff swallow than they did in the 1980s, according to a new study that suggests the birds have evolved shorter wings to pivot away from oncoming traffic.
The adaptation is important for the birds’ survival given that they nest by the thousands under bridges and overpasses there, noted Charles Brown, a biologist at the University of Tulsa in Oklahoma who regularly drives those same roads to and from a nearby research station.
"The decline in road kill is very real," he told NBC News. "What’s causing it is more a matter of speculation."
He and colleague Mary Bomberger Brown from the University of Nebraska, Lincoln, know for a fact that the bird population overall has shorter wings today than it did when the researchers first started studying it in 1982 and the road-killed birds have longer wings than average.
Given that traffic, if anything, has increased over the years, scavengers that eat road-killed birds have not increased, and the bird population itself is much larger than it was when they started their observations, the most likely explanation for the decline has to do with the shorter wings.
"A shorter wing confers greater maneuverability in flight and so if you are more maneuverable, you can turn more rapidly, you can pivot away from something," Brown explained.
He noted that a shorter wing allows swallows to take off in a more vertical than horizontal fashion.
"If you are sitting on a road and car is coming, it is probably best to fly up as quickly as possible rather than take off flying ahead of the car because you are probably not going to make it," he said. "So, the idea is that the wing length indicates the relative ability of these birds to avoid collisions."
Nimble flight, Brown added, is also key for swallows to be able to catch their prey — insects that zig-zag. It's possible that the birds evolved shorter wings to better catch insects associated with corn crops. Much of the surrounding prairie has been converted to agriculture in recent decades.
Evolution, he noted, would favor birds best adapted to catch the available food. Swallows are also known to learn behaviors and it could be that the birds that learned to avoid cars have survived and passed on their genes.
Whatever the reason, said Brown, the wings have indeed gotten shorter.
"I don't think necessarily road mortality is the sole cause of this," he said. "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."
The findings are reported today in the journal Current Biology.
John Roach is a contributing writer for NBC News. To learn more about him, check out his website.