Christie Wilcox, writing at Science Sushi:
Long wings are a benefit in certain environments. Longer, pointier wings provide increased lift and less drag, making it easier to spend lots of time in flight — a huge plus if, like the cliff swallows, you migrate long distances seasonally. A lot of birds, including most swallows, have long wings for their body size because they spend a lot of time catching flying prey like insects, and longer wings reduce the effort needed to glide and maneuver in the air. But while short wings are less efficient once in flight, they provide a bigger boost from the ground, allowing birds a near-vertical takeoff. When you’re trying to jump out of the way of a speeding car, the faster you can get out of range of the vehicle, the better.
Not only did the team find that the roadkill birds had longer wings, they found that over time, the overall wingspan of the cliff swallow population in Nebraska has been declining. While it’s possible that other factors are influencing this change, the research pair think the selective pressure to avoid cars is the real driver.