Birds may be evolving in the face of safety threats posed by cars, according to Charles Brown, a researcher at the University of Tulsa in Oklahoma. He reports seeing shorter wing spans—the better to take off from a road quickly and dart away when a car is approaching—among the cliff swallows in his area.
Traffic kills an estimated 80 million birds across the United States every year, and cliff swallows are especially vulnerable due to their frequently nesting on road bridges. Swallows roadkills in Oklahoma have been declining since the 1980s, however, and it’s not because there are fewer birds: The numbers of roadside nests have risen.
Brown says that the explanation may lie in the physical changes in the birds that he has been documenting for the last 30 years. Each year, the researcher has been collecting dead swallows from the Oklahoma roads. The last few generations, he says, exhibit shorter wings, which offer two distinct advantages: Easier vertical take-off, and more maneuverability. Shorter-winged birds can take to the air more quickly when a car is coming, and they can dart right or left in a split second once airborne to avoid being struck in mid-flight by the oncoming car’s grill.
Human activity is not necessarily the only change agent at play, however. Harsh weather can force birds to adapt, also, Brown notes, citing as an example an unexpected cold spell in May 1996 that sapped the birds’ food supplies, resulting in half the nesting population starving to death. The average wing lengths dropped markedly. Brown suggests that in this case, the swallows with shorter wings would have been more likely to survive because they were more adept at catching insects while in flight.
This wouldn’t be the first case of human activity shaping animal evolution. Studies of fish populations in many locales have found that when human fishing depletes schools’ numbers to dangerously low levels, the young start maturing at quicker-than-normal rates. Herring in Norway, pilchard in western Africa, and chum salmon in Japan are a few of the many fish species that show this trait.
Populations of another bird species, the Galapagos finch, are essentially evolving backwards as a consequence of human development. Researchers note two of the island’s finch populations that had long ago diverged—each population bearing slightly different markings—are now re-converging as one. The driving force: the replacement of their natural food sources by manufactured feed from birdfeeders.