The Economist explains

Have swallows evolved to cope with cars?

Natural selection may be at work in American cliff swallows

By E.H.

BIRDS have a habit of surprising people. Pigeons use three senses unavailable to humans, forming a far richer picture of the world than humanity can imagine. Certain species of birds have displayed signs of sexual-role reversal, to the bafflement of evolutionary biologists. And in Keith County, Nebraska, a recent study of American cliff swallows produced some surprising results. What's going on?

The American cliff swallow, as its name suggests, likes to build its hardened-mud nests on cliff faces. These days the birds are also happy to build their nests on the sides of road bridges. These offer potentially useful extra nesting sites for cliff swallows, but pose additional dangers, because birds may be killed by cars if they land on the road beneath the bridge. For 30 years Charles and Mary Brown of the Universities of Tulsa and Nebraska-Lincoln have been collecting and measuring cliff swallows killed by cars and lorries. When they began their study in 1982 they were picking up about 20 birds a year that had been killed by vehicles. By 2012 that had fallen to four. At the same time, the number of nests in the area of study had risen from 10,000 to 25,000.

One possibility is that individual birds had learned that vehicles were dangerous and changed their patterns of behaviour accordingly. But the Drs Brown think that at least part of what is happening is natural selection. They found that the birds killed by vehicles had longer wings, on average, than the population at large. Shorter wings allow a more vertical take-off, which enables a bird to get out of the way of an incoming vehicle more quickly. There would therefore be selective pressure in favour of swallows with shorter wings. They would be more likely to survive into adulthood and produce offspring with the same trait.

Since they began their research, the Browns have found that the average wing length of local swallows has fallen from about 10.9cm to 10.7cm. This change in wingspan suggests that natural selection could indeed be at work—and that, when faced with new threats from humankind, these birds might be evolving in response.

• What else should The Economist explain? Send us your suggestions.

Discover more

How to define artificial general intelligence

Academics and tech entrepreneurs disagree. A court may soon decide

Gaza could face a famine by May. What does that mean?

Some parts of the strip are already experiencing “catastrophic hunger”


What is the Islamic State Khorasan Province?

The group that claimed responsibility for the Crocus City Hall attack is a growing threat to Russia—and the West