Any birder who has driven over a bridge almost anywhere in North America has undoubtedly noticed that Cliff Swallows have readily taken to those man-made structures as nesting sites. But a study, recently published in the journal Current Biology, of Cliff Swallows in Nebraska has shown that the act of choosing these sites over the last 30 years is selecting for traits that better allow the birds to avoid the traffic which is an occupational hazard of nesting so close to roads.
The researchers, Charles Brown and Mary Bomberger Brown of the University of Tulsa and the University of Nebraska respectively, first noticed something interesting was going on when they realized that the frequency of road-killed swallows over the 30 year time frame had declined rather dramatically while the population of birds nesting under the bridges, and the traffic on the road, had increased. If natural selection favors those birds that learn to avoid cars that might be expected, but the researchers noted, through thirty years of banding nesting swallows and salvaging road-killed birds, that wing length in the birds killed by cars was significantly longer than the population at large.
They concluded that those birds with shorter wings were more maneuverable, particularly when taking off from the road ahead of approaching cars, and that the passage of cars on the road was selecting against those birds with longer wings.
[Charles] Brown says that there is evidence that shorter wings make the animals more agile: “they can make a 90º turn more rapidly”, he says. That would help the birds to dodge traffic as they exit or enter their nesting sites, or take off from the pavement, Brown explains. And that in turn would enable them to survive and produce more short-winged offspring.
Really cool stuff, and evidence of not only the amazing plasticity of birds but of the ability of some species to adapt to disturbed environments, a trait that may serve them well in an increasingly difficult world.