Sometimes research gives you answers to questions you didn’t even know you had when it is well designed. In this case researchers in Nebraska looking into the social behavior of cliff swallows, an especially communal type of bird with interesting implications for the selection of altruism, realized that they were seeing a lot less dead birds hit by cars and looked into why that might be happening with data they already had.
Cliff Swallows hanging out under a highway in Texas
CR Brown and NB Brown. Published 2013 in Curr. Biol. doi:10.1016/j.cub.2013.02.023
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.
This all makes for a pretty neat demonstration of evolution by natural selection creating shifts in population features in response to new pressures, namely very fast very deadly cars.