Three years ago the mayor of London, the Right Honourable Ken Livingstone, proposed a law that threatened, in one fell swoop, to eradicate all traces of over 400 years of British history. The mayor, after receiving a cleaning bill for £140,000, officially banned pigeons from Trafalgar Square—one of Western civilization’s most prominent meeting places, and most notorious of pigeon factories. After banning food sellers, fining seed-scattering tourists, and installing trained hawks, the Brits managed to reduce their nearly 5,000-strong pigeon flock to a scant 400 feral birds. Trafalgar Square, I should mention, is roughly the size of two football fields.
For the vast majority of animal species, the immigration of Homo sapiens into a region is an event that often ends in usable habitat loss and local population extinction. But for a few mysteriously hardy species, the detritus of human society is just what the doctor ordered. Yes, the dodo didn’t fare so well, but those pigeons and squirrels seem to be doing all right. So what is it about pigeons? What factors allow some species to thrive in human-dominated environments while others die out?
In nature, those organisms most able to adapt will often survive environmental changes (or, as in the case of cockroaches, those least susceptible to change). This is the principle of natural selection. But the differences between city and country are so dramatic that they often seem to make this trend irrelevant: even the most adaptable arboreal species will find it impossible to survive in a city bereft of the food and shelter that trees provide. But ecological systems are hardy, wherever they may exist, and if there are sufficient resources for life, there will arise some organisms able to exploit them.
For the domesticated pigeon, also known as the rock pigeon and the “sky rat,” the modern city is a paradise of resources. The feral offspring of Europe’s once-prized carrier pigeon fleet (used historically to annoy friends and neighbors until the invention of the cell phone), the modern city pigeon traces its lineage back to cliff-dwelling doves native to the coasts of southern Europe. Because they favor tall, rocky perches for their nests, rock pigeons are uniquely designed to live comfortably on almost any building in any city. This is no small feat considering that the lack of suitable territory is one of the main deterrents to natural life in the city—and it is no coincidence that one of the only large predators found in cities is also a cliff nesting bird: the peregrine falcon.
In addition to having unfettered access to prime realty, the pigeon faces little competition for food resources. In nature, the rock pigeon breeds only once a year, sometimes twice, when food is abundant enough to support the burden of egg production. In the city, however, food—in the form of grass seeds, trash scraps, and handouts—is available and abundant year round. Urban rock doves have been known to produce up to six clutches a year, usually laying at least two eggs per clutch. The combination of abundant food and nesting territory, and lack of predators (rats, the other abundant city species, have a hard time scrambling onto eves and roofs), allows the rock pigeon to dominate the urban environment.
Though resources like food and nesting space are obvious regulators of urban-survivorship, the processes that constrain life in the city, we are increasingly coming to learn, are as complex as those of any natural habitat. They are even perhaps more so, as a new study, published in last month’s edition of Current Biology, has hinted. The study suggests that there are species living in our cities that have adapted to odd constraints in ways we might not expect, constraints our pigeon friends may never have encountered. Urban song birds, it now seems, are adapting.
Specifically, the song birds that flourish in large cities have adapted their songs to survive the conditions of the city. Hans Slabbekoor and Ardie den Boer-Visser, two researchers at Lieden Univeristy in the Netherlands, compared the songs of great tits (yes, haha) living in the wild with the songs of their cousins living in urban environments. They discovered that across the board—in cities as varied as London, Paris, and Prague—the urban great tits sang notes with dramatically fewer low-frequency components than their cousins (of the same species!) in the wilds. In other words, they developed their songs to avoid those frequencies most likely to be obscured by ambient city noise, such as the low-frequency rumble of automotive traffic. These birds use song to attract mates and ward off potential competitors, so singing a song that would be masked by city noise would not only be a waste of energy but also a significant reproductive handicap (you can’t woo me if I can’t hear you). Those birds whose songs naturally had fewer low-frequency components (each bird sings a unique, slightly different song) probably gained an enormous reproductive advantage over birds whose songs were obscured by city noise. This advantage could have led to the population differences now observed between wild and city-dwelling great tits, as over several generations the offspring of birds who lacked low-frequency song elements would predominate in the few useable spaces (often trees and bushes) in the city.
The great tit, which is generally quite abundant in big cities, has shown that it can adapt its song to flourish, but other song birds with more complex, or less malleable, songs may have been unable to make the shift. While some abundant species, such as the pigeon, have had to only slightly adapt to the urban environment, it is undeniable that human centers produce incredible, and often unpredictable, selection pressures on species. Perhaps in the future there will be species suited only for life in the cities, species that have diverged so greatly from their wild counterparts that we will have to categorize them as an entirely different species. Perhaps soon the “urban habitat” will be considered a reasonable habitat zone—though maybe not one that we will ever see exhibited behind glass at natural history museums.
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