Millions of birds die in the US each year as they collide with moving vehicles, but things have been looking up, at least in the case of cliff swallows.
The findings, reported in the Cell Press journal Current Biology on March 18, show that urban environments can be evolutionary hotspots.
“Evolution is an ongoing process, and all this—roads, SUVs, and all—is part of nature or ‘the wild’; they exert selection pressures in a way we don’t usually think about,” said Charles R. Brown of the University of Tulsa.
Brown and his colleagues, including Mary Bomberger Brown from the University of Nebraska-Lincoln, have been studying cliff swallows in Nebraska since 1982. The birds there build clusters of mud nests attached to vertical walls under bridges, overpasses, or railroad tracks, often in colonies of thousands.
Those road kill surveys now reveal a sharp decline in mortality over the last 30 years, a drop that can’t be explained by declines in the bird population or in traffic volume. The birds that continue to die on the roads are those with longer-than-average wingspans.