BERLIN - How fast is evolution? Pretty fast, according to a recent study by researchers at the University of Tulsa published in the scientific journal Current Biology.
An estimated 80 million birds are killed by cars each year. In Europe too, millions of birds die the same way.
But over the past 30 years, the number of cliff swallows (Petrochelidon pyrrhonota) killed by cars in southwestern Nebraska has shrunk significantly, according to Charles R. Brown and Mary Bomberger Brown. The two researchers have been studying the birds since the 1980s and published their findings in a paper entitled “Where has all the road kill gone?”
In southwestern Nebraska, cliff swallows nest in large colonies under highway bridges and overpasses. In the past that meant that they often flew into cars. During their observations, the researchers noted that this was happening less and less frequently and looked into possible reasons behind the phenomenon.
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Nests of South African swallows under a bridge - Photo: NJR ZA
They couldn’t substantiate the most obvious causes. During the time frame, traffic had not decreased. In fact the Browns recorded an increase in larger vehicles such as SUVs, so that – because of the larger surfaces of such vehicles – the danger for the birds actually went up. They thus excluded more favorable traffic conditions as a cause for the lower road kill rates.
The global number of birds did not decrease either – on the contrary, the birds apparently enjoyed developing their highway nesting area.
Examinations of swallows that had been killed by cars showed that they generally had a larger wingspan than other swallows in the colony. According to the researchers, birds with the wider wingspan have a tougher time avoiding oncoming cars than those with a shorter wingspan, which allows the bird to turn more quickly.
Not keeping all the eggs in one basket
“One possible explanation is that selection has favored individuals whose wing morphology allows for better escape,” says the paper: “Longer wings have lower wing loading and do not allow as vertical a takeoff 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.”
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Over the decades, natural selection has favored birds with shorter wings. Since they don’t die as frequently in collisions they have more reproductive success than longer-winged swallows.
“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," says the study.
"Regardless of mechanism, the drop in traffic-related mortality over 30 years suggests that researchers should consider the possibility that road mortality in other species may change temporally and exert selection,” the researchers write. Good news for researchers whose research subjects are often the victims of traffic accidents.
Cliff swallows are also interesting to scientists for other reasons – they have a relatively novel way to insure the survival of their offspring. Female cliff swallows initially lay all their eggs in one nest. After some time has elapsed they take one of the eggs and transport it to the nest of another female. This egg is a form of “insurance” so that if their own nest is destroyed, chances are great that their genes live on in at least one chick.