Evolution works pretty fast sometimes. Biologists Charles Brown of the University of Tulsa in Oklahoma and Mary Bomberger Brown of the University of Nebraska-Lincoln have found that over a period of just 30 years, the cliff swallows (Petrochelidon pyrrhonota) of southwestern Nebraska evolved shorter wingspans that help them avoid getting hit by cars. Their study appears in Current Biology.
The discovery is a byproduct of a long-term study of the social behavior of the swallows, which nest in colonies beneath horizontal overhangs. They attach their gourd-shaped mud nests to sites like highway bridges, overpasses, and the concrete beneath railroad tracks.
In the 30 years of their study, the researchers drove the same roads, year after year, to check on the birds, stopping to pick up dead swallows whenever they found them. But they found something intriguing: The swallow population as a whole increased over that time, but the number of birds that became roadkill fell. In addition, the birds that got hit by cars had longer wings, on average, than the population as a whole, and the difference in wing length became more pronounced over time.
Factors like weather, changes in the prey population, or learning could explain either the difference in wing length or the decline in roadkill, but not both. Instead, the researchers say that natural selection likely favors birds that have a wing morphology that allows them to escape ongoing vehicles.
“Longer wings have lower wing loading and do not allow as vertical a take-off as shorter, more rounded wings,” the researchers write. “Thus, individuals sitting on a road, as cliff swallows often do, who are able to fly upward more vertically may be better able to avoid or more effectively pivot away from an oncoming vehicle.”
Image credit: Current Biology, Brown et al.