A few years ago, Anders Pape Møller from the University of Paris-Sud walked through the small suburban town of Orsay, France, counting all the birds he saw or heard. He walked through built-up urban areas, and through forest and farmland. He found that Orsay’s birds were congregating largely in the urban zones. He found 77 percent of them within a hundred meters of the nearest house. When he repeated the census in a similar town in Denmark, he found the same thing: 87 percent of local birds were sticking close to humans.
That might be surprising to some. It’s common to think of ourselves as separate from nature. Our world of concrete, glass, brick, steel, buildings, and roads seems distinct from that of mud, trees, and grass—and it’s surely the latter that attracts wildlife. And yet, cities have existed for tens of thousands of years, and they are thriving natural ecosystems.
It’s not just pigeons, raccoons, rats, and sparrows, either. We’re talking boars in Berlin and baboons in Cape Town, peregrine falcons in London, and a new species of frog hiding in New York. The study of such wildlife—urban ecology—is a huge and growing field because cities are not separate from nature; they are part of nature. The upcoming sequel to the landmark series Planet Earth, which examined each of the planet’s major habitats, will include an episode on cities as well as the usual mountains, deserts, and grasslands.
But urban living has its disadvantages, especially for birds. The din of traffic, industry, and crowds can drown out the calls and songs, forcing vocal bird species to change their tune, shout louder, or simply move to the country. So what persuades some species to stay? What do they get out of proximity with humans? Møller had an idea: Perhaps we inadvertently protect birds from parasites.
Many bird species are targeted by cuckoos and other brood parasites—birds that fob their young off onto other species. A cuckoo chick is bad news: it means extra responsibility for the surrogate parents, and it also kills their actual young by jettisoning them from the nest.
But cuckoos, it seems, rarely set up shop near human settlements. They have plenty of potential hosts, including swallows, wagtails, and starlings, but for some reason, they tend to keep away. Møller found this pattern by looking at a large European database, with more than 35,000 cases of cuckoo parasitism. And his colleague Wei Lang from Hainan Normal University found the same pattern by studying Oriental reed warblers in China.
The team also found evidence for it during their French and Danish walks. They saw that the average bird perches around 67 meters from the nearest human house, but the average cuckoo stays around 204 meters away. The cuckoos are more skittish, too. Most songbirds will let humans get within seven metres of them before flying off, but for cuckoos, this “flight initiation distance” is 30 meters.
So towns and cities might be refuges from parasites. They’re places where a bird can raise its young secure in the knowledge that its young is actually its young.
There are probably other benefits too. Cities provide rich sources of food, and often safety from predators that are deterred by humans or our domestic pets. Møller found some evidence for this, too. In 2012, he showed that, like cuckoos, hawks and other predatory birds are also skittish around humans. On average, they take off if people get within 54 meters, which is eight times farther than their songbird prey allow. This means that humans create safe zones around ourselves that small birds will tolerate and their predators will not.
Far from creating worlds that are inhospitable to wild things, in this case we have actually produced refuges for many.