Cliff swallows, as their name suggests, like to build nests on cliffs and other rocky outcroppings. They also like building their nests on bridges and overpasses, and sunbathe on warm roads. This puts them in the path of traffic, and adds thousands of swallows to the nearly 80 million birds killed by cars each year in the US. Swallows in the state of Nebraska, however, appear to be getting wise to the ways of the highway – or at least their genes are.
Researchers at the University of Nebraska have been collecting swallow bodies along several highways for the last thirty years. Not only have the total number of fatalities decreased over this time, but the wing length of the birds has also been decreasing. The swallows, it seems, are evolving to become more nimble. Shorter wings makes it easier to take off vertically or to quickly maneuver around vehicles. Time to add "vehicular selection" to the sub-categories of "natural" selection.