According to Current Biology, researchers Charles Brown and Mary Bomberger Brown have been paying close attention to the swallow population in this particular section of Nebraska. They've found that the birds measured now have shorter wings than did the birds that were first studied back in 1982. What's more, there are a smaller number of roadkill birds found in the area – despite increases in both swallow population and traffic since the study began. No increase in roadkill-eating scavengers has been found either.
That evidence suggests that swallows with shorter wings are better able to maneuver out of the path of oncoming vehicles. Roadkill birds found these days have longer wings than average, too, which seems to back up the speculation. Scientists also believe that the shorter wings and increased maneuverability has allowed the cliff swallows to be more adept at catching insects that live in the increasingly frequent corn fields around the area, too. Brown says that evolution would likely favor the birds that have adapted to better catch their dinners, as well as passing along the genes that allow them to avoid killer cars. Science, man!