Cliff Swallows adapting to life on the road, study shows

Cliff swallows nest under a bridge on US 90 in Louisiana. (Photo by Carol Foil/ CC BY-NC-ND 2.0)

Cliff swallows could be the Evel Knievels of the bird world. They have an affinity for living in extreme places—cliffs, buildings, under bridges, in the crevasses of railroad tracks—and they appear to be getting better at cheating death.

Every year an estimated 80 million birds, of all species, are killed by vehicles in the United States, and those numbers are likely rising, say experts. However, a new report in Current Biology shows one population of cliff swallows in Nebraska is bucking that trend, possibly by adapting to an increasingly urban environment at a breakneck pace.

During a span of 30 years scientists recorded body measurements of cliff swallows that were killed by cars along the same route in southwestern Nebraska, and compared them with swallows from the same population that had been caught in mist nests. The researchers found the mist-netted birds had significantly shorter and sleeker wings, suggesting those animals might be advantageously adapted for pivoting and dodging oncoming traffic.

At the same time, the number of road-killed birds plummeted—from 20 birds a year at the start of the study in 1983 to two last year—while the number of cliff swallow nests mushroomed from 5,000 to 25,000. 

The researchers say there could be multiple adaptive strategies at work. They note that cliff swallows are tremendous observers, so they may have learned to avoid vehicles over time by watching what happened when birds didn’t dart away from cars and buses. Perhaps natural selection is favoring intellectual birds, which then pass their knowledge onto their offspring.

Taken as a group, the findings suggest that cliff swallows have undergone “rapid evolution,” says Charles Brown, an ornithologist at the University of Tulsa and the lead author of the paper co-published with his wife, Mary Bomberger Brown, an ornithologist at University of Nebraska-Lincon.

With the help of many undergraduate students, the Browns spent three decades scraping bird bodies off roads and examining swallow specimens collected from bird banding stations.

“Over the past year I had begun to intuitively think that we had begun to find less roadkill, so it was then very exciting to go back and find that the math verifies what we were seeing,” says Brown.

This study is significant in that it opens the possibility that other species may be undergoing similar evolutionary changes. As humans continue to sprawl into rural environments, species may face two inevitable fates: adapt or be road kill.

“It’s probably difficult to generalize the same trend of adaptability in other species that often are road kill, but I think this would be a common phenomenon among a number of bird species that live in urban environments,” says Brown. “With other road kill species, like turtles and snakes, this may require a much more drastic change in their behavior.”