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.