Modern transportation has not been kind to cliff swallows, but a new study in Current Biology shows that evolution may be helping to solve a roadkill epidemic in their ranks.
The birds build their mud nests on vertical walls and they find highway overpasses to be an irresistible place to roost. The trouble is, when they fly away from home, often it's right into the grill of an oncoming tractor trailer: One more dead swallow among the estimated 80 million birds killed in collisions with vehicles in America each year.
For the last 30 years biologists Charles Brown and Mary Bomberger Brown have counted the number of road-killed cliff swallows along the roads in southwestern Nebraska. They noticed that the number of dead birds has been declining, and they wondered why. Was it that the swallow population had thinned? Or that traffic volume had decreased? Or that scavengers were making quicker work of the carcasses? Nope.
Instead, they realized, something more dramatic had taken place. Over 30 years, the average cliff swallow wingspan has shrunk, and cliff swallows with shorter wings are able to take off faster, which makes it easier for them to get out of the way of your 65mph family vacation. They also observed that cliff swallows with longer wings tend disproportionately to end up dead on the side of the road. This suggest (although the biologists can't say for sure) that in a relatively short period of time, evolution has selected for cliff swallow attributes better suited to life in an interstate age.
H/T Eureka Alert.
Image courtesy of Current Biology, Brown et al.
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