Cars are driving the evolution of birds in Nebraska, leading to birds with shorter wings to take off more quickly and avoid onrushing vehicles, scientists say.

Of the 80 million birds killed in the United States by traffic each year, cliff swallows are particularly vulnerable because many have taken to building their nests on road bridges over streets and highways, researchers say.

However, Charles Brown of the University of Tulsa, who has been counting dead swallows for decades, said roadkill numbers have steadily declined since the 1980s even though the number of swallows nesting on roadsides has risen, NewScientist.com reported Monday.

Birds killed by traffic have been found to have longer wings than living birds caught in nets for research, researchers said.

Shorter wings may improve the birds' ability to make quick vertical take-offs and improve their maneuverability, they said.

"Everything fits with the idea that it's vehicular selection," helping them avoid dying on roads by taking off quickly and darting away from cars, Ronald Mumme of Allegheny College in Pennsylvania said.