Gary Hochman at KQED tells a fascinating story about how humans have impacted the evolution of cliff swallows to become smaller, but more agile.
In a 30-year study of cliff swallows – one of the longest running evolution studies in the world – University of Nebraska ornithologist Mary Bomberger Brown has found that construction of highways and interstates, along with concrete bridges and overpasses, had drastically altered the size, shape and behaviors of cliff swallows.
By chance, their study coincided with a sweeping change in roadway design. A nationwide boom in highway and interstate construction was replacing old wooden trestle bridges with new concrete bridges and overpasses. Bomberger Brown soon realized that the swallows abandoned nesting on cliffs of loose, crumbly sandstone in favor of firm concrete structures that offered overhangs sheltering the birds from the elements.
"We built them a better cliff, says Bomberger Brown, "and the birds flocked to them." As the highways, expanded east and west, the birds followed suit. "You can now see them across the country," she adds. "They've just followed people in their roadways, so humans have actually expanded their range – from ocean to ocean." To track the birds, the Browns placed metal leg bands on the swallows to identify them. And to learn how the birds compare, they capture the birds to measure their bodies, beaks, wings, feet, and tails.
And as roadways spread, so did agriculture and insects. "It's a mutualistic relationship," explains Bomberger Brown. "We've given them more food. We've given them places to nest. The birds provide pest control. They live with us and they've become part of our landscape."
After finding dead cliff swallows along the new highways, the researchers began measuring the birds. Then years later, measured more that were killed by a freak storm.
What they found was a revelation:
Measuring each dead bird and comparing them to the birds that remained, the Browns made a surprising discovery. The birds that died had shorter bodies and longer asymmetrical wings. But the birds that lived had larger bodies and shorter wings.
The survivors' shorter wings made them more acrobatic, like a fighter plane. They were able to twist and turn, dodging oncoming traffic, especially larger vehicles like SUVs and Mac trucks. The road kill birds had been less agile.
As Hochman explains, typically we expect evolutionary changes like this to take thousands, or even millions of years, but thankfully for the cliff swallows, they were able to adapt astonishingly fast to match the rapid changes to the American landscape.
Of course, this doesn't mean that our massive system of roads and highways are good for all wildlife – often, the opposite is true – but in this case, it is nice to hear that the swallows have adapted to thrive in our faster, busier and more-congested world.
Read the rest of this report at KQED.