A 30-year study of birds and roadkill may have given us a look into how animals respond to man-made changes in the environment. According to a study published last week in Current Biology, an area in southwestern Nebraska has seen a marked drop in cliff swallows found as roadkill. At the same time, the average wing length in the group decreased, with one exception: birds who were found dead on the roadside had a wingspan that was "significantly larger than in the population at large." One plausible explanation? In areas with roads, cliff swallows tend to nest in overpasses, bridges, or culverts, putting them at risk of death. But those with shorter wings can manage a more vertical takeoff, getting them out of the way of incoming vehicles.

If birds with longer wingspans are more likely to be killed, smaller ones could have a reproductive advantage, leading to a larger shift in the population; correspondingly, as long wingspans decline, birds could be less likely to end up as roadkill, explaining the decline. While there are always other possible explanations for finding fewer dead roadside birds, authors Charles and Mary Brown don't believe the populations of scavengers have varied, and other changes would logically point to more dead birds, not less: vehicle traffic either stayed the same or increased, and vehicles are likely to have gotten larger since the study started in 1982.

An alternate hypothesis for finding fewer dead birds is that over 30 years, they've learned to avoid traffic. The authors say they couldn't evaluate this, but that younger birds weren't more heavily represented among the dead ones — which one might expect if the swallows were exhibiting a form of learning. They, however, believe other factors are at work in the birds' declining wingspan. Changing weather or insect patterns, for example, could lead to smaller wingspans. For now, though, it seems possible that even if many people think of man-made structures as "unnatural," we're playing a role in the birds' natural selection.