High-Speed Evolution: Cars Driving Change In Cliff Swallows

I imagine that adjusting to life around humans, with all our buildings and fast-moving transport mechanisms, is tough for a bird. It’s estimated that some 80 million birds are killed in motor vehicle collisions every year, and with an ever-growing population of people driving around and paving roads in more remote areas, things must be getting harder and harder for the animals we share our world with. But, the American Cliff Swallow (Petrochelidon pyrrhonota) isn’t one to let people ruin the neighborhood. More and more, their huge nesting populations can be found in man-made structures like bridges and overpasses, and have even become cultural fixtures in areas like California. Their new nesting sites allow them to survive even as their former habitat disappears, but it comes at a cost: by living near roadways, the birds are more at risk than ever of being on the wrong end of an oncoming vehicle.
Charles and Mary Brown have been studying the cliff swallows in Keith County, Nebraska for the past 30 years. The ultimate goal of their research is to understand why colony sizes vary, and together, the two have studied everything from parasites and diseases to social interactions between birds, with over a hundred publications between them. The American Ornithologists Union describes their three-decade long research on cliff swallows as “one of the most outstanding and most complete studies of any avian species .” Every year, the team monitors the birds and collects any dead ones they can find for further analysis. Slowly, they noticed something strange: even though nests under overpasses and on bridges should have put more swallows in harms’ way, over time, fewer and fewer swallows are winding up as roadkill.

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