Date of this Version
Current Biology 23:6 (March 2013), pp. R233–R234.
An estimated 80 million birds are killed by colliding with vehicles on U.S. roads each year , and millions more die annually in Europe  and elsewhere. Losses to vehicles are a serious problem for which various changes in roadway design and maintenance have been proposed . Yet, given the magnitude of the mortality reported for some species , we might expect natural selection to favor individuals that either learn to avoid cars or that have other traits making them less likely to collide with vehicles. If so, the frequency of road kill should decline over time. No information is available for any species on whether the extent of road-associated mortality has changed . During a 30-year study on social behavior and coloniality of cliff swallows (Petrochelidon pyrrhonota) in southwestern Nebraska, we found that the frequency of road-killed swallows declined sharply over the 30 years following the birds’ occupancy of roadside nesting sites and that birds killed on roads had longer wings than the population at large.