Every year for the past 30 years, researchers have been acting like urban Darwins, observing cliff swallows in Nebraska and surveying how they die. The swallows often build nests under bridges or other well-trafficked areas and so, occasionally, get hit by cars. The researchers tracked birds who've died by car, and then compared them to birds who accidentally died some other way. They've found that, during the last three decades, the swallows have been dying less by car, and it's not caused by the number of birds in the area or number of cars on the road. Something else is up, the researchers surmised.