Researchers Charles R. Brown and Mary Bomberger Brown have been driving down the same roads in southwestern Nebraska for 30 years collecting and studying dead Petrochelidon pyrrhonota, or cliff swallows that make their mud homes on the undersides of bridges and overpasses. In all that time, the incidence of road kill among the species has plummeted dramatically. But the two most obvious explanations for the decline – a drop in either traffic volume or bird populations – don't seem to fully explain it.
In new research published in the journal Current Biology, the researchers propose instead that these swallows seem to have evolved in the past three decades to dart out of the way of passing traffic: They now have shorter wingspans, handy for quick take-offs and turns. Those longer-winged birds that haven't yet made the evolutionary transition appear more likely now to be hit by cars (the researchers know this by comparing road kill to birds that died of other causes).
We've heard before of urban birds learning to sing over the noise of traffic. But this evolutionary maneuver seems to go even one step further, altering the very shape of this wildlife. This news may be depressing or encouraging depending on how you look at it. On the one hand, the car has become so dominant in our landscape that even swallows must evolve to coexist with it; on the other, at least they're finding ways to survive.
The researchers leave open the possibility that some other explanation may be at work here in the declining mortality of these swallows. But the data also suggest that wildlife may yet find a way to live with our busy roads even when we're not actively doing much to help them out.
Both images courtesy of Current Biology, Brown et al.