Cars are incredible inventions. They allow us to travel at high speeds to almost anywhere we want to go. But for animals, cars are a brand new danger. Never before have they had to avoid a giant piece of metal hurtling across the landscape at high speeds, and this new obstacle kills millions of animals every day. But new research suggests that perhaps, for some animals, evolution might be kicking in and helping them adapt to avoid cars.
The research comes from the University of Tulsa and looks at cliff swallows in particular. In a study published in Current Biology, the researchers show that since 1982, when the team began studying the swallows, their wings have gotten shorter. In that same period of time, fewer birds have fallen victim to oncoming traffic. The researchers conclude that this shorter wing span has helped the birds maneuver in the air more quickly and pivot away from oncoming traffic.
In 2005, High Country News summed up some road kill statistics: across 4 million miles of roads in the United States, there are 253,000 animal-vehicle accidents and 1 million vertebrates run over each day. That’s one every 11.5 seconds. The paper here estimates that 80 million birds are killed by cars every year. Cliff swallows are particularly likely to be hit because they build their nests on cliff like surfaces. Sometimes those surfaces are actual cliffs, but other times they’re bridges or overpasses. And the birds also have a pesky habit of sitting on roads near their nests, which put them in direct danger of being hit by cars.
So for the last thirty years, the researchers at the University of Tulsa have driven a set of roads in Nebraska, collecting little bird bodies. Not only have those bird bodies decreased, but the population’s wing span has decreased with it. The paper writes:
Our results indicate that these birds since then have become increasingly less likely to collide with cars and that road mortality is not indiscriminate. One possible explanation is that selection has favored individuals whose wing morphology allows for better escape. Longer wings have lower wing loading and 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 .
These changes in death rates aren’t explained by changes in traffic patterns or population of the birds, the researchers say. And they’re calling this change in wing span “vehicular selection.” But it might not be the only force at play. New Scientist writes:
However, Brown says that encounters with traffic may not be the only force at work. After a particularly cold May in 1996 killed about half the nesting population through starvation, wing lengths dropped markedly, perhaps because birds with shorter wings were better able to capture the remaining insects still on the wing.
Tthese birds aren’t the first animal to exhibit evolution to avoid humans, says New Scientist. Fish mature more quickly due to fishing, and finches are evolving back into one species due to the bird feeders. And now it seems that our love affair with the road could mean a whole new kind of swallow.
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