In October 1908, after several false starts, one of the world's most effective predators finally found a niche where its population could begin to boom. A century later, this ruthless hunter has spread throughout the world and is known to prey upon everything from insects to grizzly bears, although it does occasionally lose when attacking the latter. Among its prime targets are birds, of which it is estimated to kill nearly 100 million every year in the United States alone1. I refer, of course, to the automobile.
The number of birds killed by automobiles, often referred to as "road kill", can be significant, particularly for vulnerable species. However, a new analysis2 shows that one type of bird – the cliff swallow in Nebraska – may be fighting back. The cliff swallow's weapon? Evolution.
Biologists Charles and Mary Brown noticed that cliff swallow road kills had steadily decreased since the 1980s, which was puzzling because neither cliff swallows nor automobiles had decreased in population. In fact, a new, more lethal automobile subspecies had become more prevalent during that time period, the sportus utilitus, which has developed a larger cross-sectional killing surface. Another interesting observation was that cliff swallow wings have steadily become shorter when averaged over the population, but that wings of road killed swallows have been growing longer.
1. Erickson, W.P., Johnson, G.D., and Young, D.P., Jr. (2005). A summary and comparison of bird mortality from anthropogenic causes with an emphasis on collisions. USDA Forest Serv. Gen. Tech. Rep. PSW-GTR 191, 1029–1042.
