From the summary:
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.In effect, swallows with larger wings were less maneuverable and were more likely than average to be killed by passing cars. In contrast, swallows with shorter wings (even by a few mm according to the article) may be more able to dodge oncoming traffic.
This puts a very strong selective pressure onto birds with shorter wingspans in bird colonies that nest on bridges and roads. The researchers hypothesized that over a long span on time the number of road kills would decline as the birds adapted.
What they didn't expect was that the change would take only 30 years! The change in wingspan was the only noticeable adaptation that the researchers reported, but it's quite possible that more subtle adaptations are involved here as well.
Via Science News