Of Swallow Wings and SUV Windshields

I often think of high-rise buildings, subways, and dams when I think about characteristic human changes to the landscape. But in terms of acreage, high-input agricultural zones have been a more widespread and comprehensive land use change in the past two centuries. This land use type includes not just the actual cornfields but also the houses, grain elevators, country highways, and small towns that support the farming. The cumulative effect has been to almost wipe out grasslands worldwide, replacing them with an ecosystem with many different pressures and opportunities than the previous one.

These changes have measurable impacts on wildlife that are often surprising. Wild populations do not merely shuffle out of the way when humans come to settle. It is always a collision. Some are killed in droves like the American buffalo. Others slowly or steadily decline over many years, like grassland songbirds. And a few special species are able to adapt and thrive in the new ecosystem.

Photo by Don DeBold

Cliff Swallows are small, handsome flycatching birds with a blue back and wings, a white belly and a red throat. They create colonies of nests on vertical surfaces made from mud they carry in their beaks. In Nebraska, biologists have been studying the habits of Cliff Swallows for over 30 years. There, the birds are not nesting on natural cliffs but the constructed ones under highway overpasses and roadway culverts. Birds with shorter wings are better able to avoid passing cars, and short wings are now the norm in these cliff swallow populations, according to Charles Brown and Mary Bomberger Brown of U. Nebraska-Lincoln and U. Tulsa. They described their findings in a paper, “Where has all the road kill gone?” published in Current Biology in 2013.

Brown and Brown compared the roadkilled birds to a sample of birds which died accidentally while the researchers used mist-netting to capture individuals for study. They saw that the roadkilled birds almost all had longer wings than the rest of the population. Over the 30-year study, fewer and fewer birds ended up on the highway shoulder, and the general population’s wings got shorter.

To understand why short wings help you dodge a Hummer, we have to consider flight physics. Longer wings lead to efficient flight and shallow, graceful takeoffs by decreasing the amount of bird mass carried by each square inch of wing. But shorter, stubbier wings allow for greater maneuverability and more vertical takeoffs. These sharply angled takeoffs, the researchers believe, are what allowed the short-winged swallows to launch away from the road surface as a car approaches more successfully than their long-winged relatives.

While the outcome has a strange beauty, the process was plainly brutal. The selection pressure was millions of unforgiving windshields. Roadway traffic kills 80 million birds a year in the U.S. alone. If you look at Brown and Brown’s figures, you’ll see that even in a moderate year, they and their students collected tens of thousands of cliff swallow corpses. And these were the cliff swallow casualties only in their Nebraska study area. Not that the butcher’s bill here is so different from that resulting from other kinds of selective pressures. Evolution works through death—new forms arise because less successful individuals die.

Yet even though we can read the cliff swallow’s story as a success of natural selection, what stands out to me is how few species have been able to adapt this well. In the same region, birds such as Henslow’s Sparrow, the Upland Sandpiper, and the Greater Prairie Chicken have faced severe declines due to the loss and fragmentation of even marginally suitable habitat. For these birds, evolution would not be a matter of changing wing length but changing their entire lifestyle, something that is probably impossible within timescales meaningful to humans. Are the opportunities that one country highway creates for Cliff Swallows balanced by the splintering of the remaining lands for declining and endangered species?

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Citation

Brown, C.R., and M.B. Brown. 2013. Where has all the road kill gone? Current Biology 23:233-234.