Shorter wings help
LINCOLN, Nebraska––Cliff swallows nesting under highway overpasses in southwestern Nebraska appear to have evolved shorter wings in only 30 years to help them avoid colliding with cars, ornithologists Charles R. Brown and Mary Bomberger Brown reported in the March 18, 2013 edition of Current Biology.
The hypothesized adaptation might also help cliff swallows to evade predators, by enabling them to make quicker take-offs.
“An estimated 80 million birds are killed by colliding with vehicles on U. S. roads each year,” the Browns began, picking a roadkill mortality estimate amounting to barely a third of the highest recently published.
“Frequency of roadkilled swallows declined sharply”
“Given the magnitude of the mortality reported for some species,” the Browns continued, “we might expect natural selection to favor individuals who either learn to avoid cars or have other traits making them less likely to collide with vehicles. If so, the frequency of road kill should decline over time. We found that the frequency of road-killed swallows declined sharply over the 30 years following the birds’ occupancy of roadside nesting sites, and that birds killed on roads had longer wings than the population at large.”
Finding fewer roadkilled cliff swallows each year from 1983 through 2012, the Browns observed that, “This result could not be explained by concurrent decreases in the cliff swallow population, because the population increased. The decline in road kills also could not be related to increases in the number of avian scavengers over time, as none showed significant increases.
“Direct information is not available for mammalian scavengers within the study area,” the Browns wrote, “although populations of those species associated with humans,” such as cats, “probably have not changed, given that the resident human population” of the study area “varied little during the study. Road-kill trends did not result from reduced vehicle traffic volume over time, which either did not change significantly or increased.”
Noticing the gradually decreasing length of roadkilled cliff swallows’ wings, the Browns realized that “Longer wings do not allow as vertical a take-off as shorter, more rounded wings. 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.
“Vehicle mortality is likely to be not the only factor contributing to the decline in wing length in this population over time,” the Browns acknowledged. “Severe weather events that cause selection on body morphology and changes in insect prey may also be responsible. Other explanations for the reduction in road kill are that swallows may learn to avoid collisions as they encounter a vehicle themselves or observe other birds flying away from vehicles or getting hit, or that risk-taking individuals have been selectively removed. We cannot directly evaluate these hypotheses, although if individuals are likely to avoid cars after a close encounter, we would expect younger birds to be overrepresented among the road kills, which they were not.”
Commented Vox Felina blogger Peter Wolf, “Neuter/return opponents sometimes argue that because the domestic cat did not evolve alongside many of the prey species found in this country, they have a distinct advantage. But if cliff swallows can evolve shorter wings to adapt to the hazards of living under bridges and overpasses—over a span of just 30 years—then why not adaptations to avoid other hazards, too? It’s too soon to say, obviously—but in light of these findings, the idea doesn’t seem far-fetched.”
About the authors
The Browns, with Stephen J. Dinsmore, are co-authors of the field guide Birds of Southwestern Nebraska. Charles R. Brown is presently a professor at the University of Tulsa, Oklahoma, while Mary Bomberger Brown directs the Tern & Plover Conservation Partnership at the University of Nebraska at Lincoln School of Natural Resources. The American Ornithologists Union in 2009 honored the Browns with Elliott Coues Award for extraordinary contributions to ornithological research.
Mary Bomberger Brown is in the same department at the same university as Aaron M. Hildreth, Stephen M. Vantassel, and Scott E. Hygnstrom, authors of Feral Cats & Their Management, a 2010 report endorsed by American Bird Conservancy vice president for conservation advocacy Darin Schroeder, which recommended “Lethal methods, such as trapping with euthanasia, kill-trapping, and shooting” to control feral cat populations.
But Brown appears to have had no part in producing the report, and is not mentioned either in The Practical Guide to the Control of Feral Cats, recently self-published by Vantassel.
“I encourage everyone to help their politicians find the moral backbone necessary to stand up for the environment and assert the right of landowners and environmentalists to protect their property and native species from the menace of feral cats,” says Vantassel in a series of disclaimers, including mention that his views “do NOT reflect the views of the University of Nebraska at Lincoln.”
The book appears to be largely adapted from articles written originally for media including Fur-Fish & Game, Fur Taker Magazine, and Trapper’s Post. Amid extensive discussion of how to kill cats, Vantassel advises special caution about killing ear-tipped cats because “Ear-tipping is a method of identifying treated cats in the free-range cat lobby groups.”
“Cats are rather good climbers of trees and even ladders. I have personally witnessed a cat on the roof of a building,” Vantassel adds, proving it by including a photo of a cat on a roof.
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