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."