The study’s authors discovered the trend after noticing that the number of vehicle-killed birds had declined over the past three decades. They suggest that the two findings provide evidence of roadway-related adaptation.

“I’m not saying that it’s all because of wing length,” says Charles Brown, a biologist at the University of Tulsa in Oklahoma and one of the authors of the study. But, he says, the shortening does support the idea that the birds are adapting to disturbed environments, as other organisms presumably are.

Together with Mary Bomberger Brown, a ornithologist at the University of Nebraska–Lincoln, Brown tracked roadside populations of cliff swallows (Petrochelidon pyrrhonota) in western Nebraska for 30 years, mostly to study the birds social behaviors within their colonies.