Collisions with road vehicles are driving a population of swallows to evolve into faster, more agile fliers with shortened wings.
Natural selection has favoured birds that are better able to get out of the way of oncoming cars and trucks, scientists in the US discovered. Survivors tend to have shorter wings that make it easier to take off quickly and pivot out of the path of danger.
As a result, the birds' wings have reduced in length by as much as four millimetres over the past 30 years, the researchers found. This in turn has led to a sharp reduction in the number of swallows killed on the roads in Keith County, Nebraska, where the study was conducted.
The findings are an example of how urban environments can become evolutionary hotspots, according to lead scientist Dr Charles Brown, from the University of Tulsa in Oklahoma. He said: "Evolution is an ongoing process, and all this - roads, SUVs, and all - is part of nature or 'the wild'; they exert selection pressures in a way we don't usually think about.
"Longer-winged swallows sitting on a road probably can't take off as quickly, or gain altitude as quickly, as shorter-winged birds, and thus the former are more likely to collide with an oncoming vehicle."
The study, published in the journal Current Biology, began in 1982 with researchers scouring the roads for dead cliff swallows, which attach their mud nests on vertical walls under bridges and overpasses. Some colonies contain thousands of birds, creating a serious traffic hazard.
Monitoring the road kill every year showed a dramatic reduction in deaths due to collisions between 1983 and 2012. The change could not be explained by reduced traffic volume, which either remained the same or increased during this period. Nor was it due to bodies being removed by scavengers such as skunks, whose numbers had declined rather than risen.
A clue to what was happening emerged when the scientists discovered that birds killed on the roads tended to have longer than average wings. In fact, the wing length of cliff swallows found dead on the roads became increasingly different from that of the population at large over the three decades.
"Average wing lengths of population as a whole exhibited a significant long-term decline during the years of the study, whereas the opposite pattern held for the birds killed on roads," the researchers wrote.
They acknowledged that other factors may also come into play, such as birds learning from "near misses" or watching others coming to grief, or risk-taking individuals with higher death rates being removed from the population. "We cannot directly evaluate these hypotheses, although if individuals are likely to avoid cars after a close encounter, we would expect younger birds to be overrepresented among the road kills, which they were not," said the scientists.