Cliff swallows that build nests that dangle precariously from highway overpasses have a lower chance of becoming roadkill than in years past thanks to a shorter wingspan that lets them dodge oncoming traffic. That's the conclusion of a new study based on 3 decades of data collected on one population of the birds. The results suggest that shorter wingspan has been selected for over this time period because of the evolutionary pressure put on the population by cars.

"This is a clear example of how you can observe natural selection over short time periods," says ecologist Charles Brown of the University of Tulsa in Oklahoma, who conducted the new study with wife Mary Bomberger Brown, an ornithologist at the University of Nebraska, Lincoln. "Over 30 years, you can see these birds being selected for their ability to avoid cars."

The Browns have studied cliff swallows (Petrochelidon pyrrhonota) in southwestern Nebraska since 1982. They return to the same roads every nesting season to perform detailed surveys of the colonies of thousands of birds that build mud nests on bridges and overpasses in the area. Along with studies on living swallows—counting birds and eggs, netting and banding individuals, and observing behaviors—the Browns also picked up swallow carcasses they found on the roads, in the hopes of having additional specimens to measure and preserve. They hadn't planned studies on roadkill numbers, but recently they began to get the sense that they were picking up fewer dead birds than in the past.

When the researchers looked back at the numbers of swallows collected as roadkill each year, they found that the count had steadily declined from 20 birds a season in 1984 and 1985 to less than five per season for each of the past 5 years. During that same time, the number of nests and birds had more than doubled, and the amount of traffic in the area had remained steady.

The birds that were being killed, further analysis revealed, weren't representative of the rest of the population. On average, they had longer wings. In 2012, for example, the average cliff swallow in the population had a 106-millimeter wingspan, whereas the average swallow killed on the road had a 112-millimeter wingspan.

"Probably the most important effect of a shorter wing is that it allows the birds to turn more quickly," says Charles Brown. Previous studies on the dynamics of flight have illustrated the benefits of short wings for birds that perform many pivots and rolls during flying and shown that shorter wings also may allow the birds to take off faster from the ground, he adds.

When the researchers analyzed the average wing length of the living birds in the population, they discovered that it had become shorter over time, from 111 millimeters in 1982 to the 106 millimeter average in 2012. The data suggested to the Browns that roadkill deaths were a major force driving this selection. Birds with longer wings would be more likely to be killed by vehicles and less likely to reproduce, the team reports online today inCurrent Biology.

The data illustrate a "beautiful trend that never could have been predicted," says evolutionary biologist John Hoogland of the University of Maryland Center for Environmental Science in Frostburg, who was not involved in the study. "We humans, because we're changing the environment so much, are adding a new kind of natural selection to these animal populations."

Few studies have looked at long-term changes in roadkill numbers, Charles Brown says, so more work is needed to determine whether similar trends hold for swallows in other areas, for other types of birds, or for mammals. "I would think that this would be a pattern that certainly might apply to other species," he says. "But there's almost nothing in the literature on historical trends in roadkills, because surveys typically last a season or two, not an extended period of years."

The new findings could also apply to birds killed by wind turbines, Hoogland adds, and they illustrate the payoff that can come with careful data collection and observation. "I think the most important lesson from this research is the paramount importance of collecting data even when you're not sure what it means or how it could lead to findings in the future."