In March 2013 John Marzluff, a veteran ecology professor, spent a few days in Yellowstone National Park. As he always does when out of doors, Marzluff counted the birds he found there: twenty-four species. Traveling to New York City afterward, he spent several hours over two days in Manhattan’s green space, Central Park. There he checked off thirty-one species. Central Park was “birdier” than Yellowstone!
Marzluff was not entirely surprised. Over the last thirteen years he and his graduate students at the University of Washington have studied what happens to birds, mammals, and other wildlife when forests around Seattle and other cities are transformed into residential neighborhoods. This engaging book tells what Marzluff and his students found out about suburbia and its surprisingly rich wildlife, and how we should respond to this abundance.
Marzluff and other urban ecologists find a gradient in bird life. A few tough survivors hang on in the urban core; the open country outside has many birds. In between—in leafy, variegated suburbia—there is the richest mixture of bird species of all. This finding is counterintuitive. One would have imagined that what he calls the “urban tsunami,” the global shift of populations into cities, would result in homogenized biological deserts with only a few starlings, house sparrows, and pigeons for bird life. That fails to take into account many wild animals’ elemental will to survive, and their capacity to adapt rapidly to new opportunities. Marzluff cautions that about 30 percent of a suburban landscape needs to be planted in natural vegetation for rich bird life to be present. But if birds can find food, shelter, and a place to rear young in and around cities, they will do so. Just ask ConEd about the monk parakeets, introduced from Argentina, that find power poles with transformers just right for their enormous colonial stick nests.
Marzluff is onto something more systematic than such anecdotal details. He is interested in suburbia as a definable form of natural habitat, just as specific in its biological features and inhabitants as a salt marsh or an alpine meadow. One major ecological characteristic of suburbs is variety. While unbroken forest can be poor in variety, the mosaic of lawns, parks, gardens, shade trees, streams, and ponds found in suburbia offers abundant opportunity for creatures able to adapt to it. This has become a matter of major ecological significance, and the subject of a new branch of study. A million new acres become urban in the United States each year, and 40 percent of Americans live in suburbs.
Another characteristic of the suburbs as wildlife habitat is its relative novelty. Cities are ancient; open country is primeval. Suburbs—extensive residential neighborhoods with low population densities and lots of vegetation—have come into existence mostly since the creation of rapid transportation, and mainly in the decades since World War II. Animals and birds have become more conspicuous in them very recently, as the suburbs push further out into bear and bobcat territory, and as the bears and bobcats find new niches to inhabit within them. Some surprisingly showy species, such as the crow-sized pileated woodpecker with its flaming crest, the red-tailed hawks that have nested on Fifth Avenue since the 1990s, and the great horned owls that nest in Pelham Bay Park in the Bronx, have returned to favorable urban spots.
Marzluff follows other biologists in dividing urban wildlife into three categories: avoiders, exploiters, and adapters. The avoiders are what one would expect: species that cannot or will not tolerate ubiquitous humans and their constructions. They leave soon after the ground is cleared for a new subdivision. Most of the larger mammals fall into this category. Therefore the gradient for mammal populations from city center to country is different for mammals than for birds. By and large only mammals that are small (rodents, moles), extremely versatile (coyotes), or sheltered from hunting pressure (deer) can cope with suburbia.
The exploiters find opportunity even in the densest human settlement. Many of these are scavengers. Marzluff doesn’t say much about this phenomenon, though it is a major aspect of urban ecology. Millions of vultures of several species inhabited Indian cities until recently, disposing of dead animals that Hindus did not eat and even of the human corpses on Parsi Towers of Silence, until the recent calamitous die-off of vultures from diclofenac, a chemical used in animal husbandry. Herring gulls and great black-backed gulls have expanded their breeding range all the way down the Atlantic seaboard of North America from Maine to Florida since 1900 to take advantage of urban trash heaps.
In the United States, the most intensively built city centers contain mostly exploiters, species that Marzluff calls the “fab five” avian “weeds”—rock pigeons (the ubiquitous street pigeons that started out as cliff-nesters in the Middle East and spread to every city on earth), starlings, house sparrows, mallards, and Canada geese. The first three (or maybe four) of these are not native to North America, but have spread to every continent.
This is a global phenomenon. Only the species differ. Australian cities have the noisy miner (that Marzluff mentions) and other city dwellers like rainbow parakeets (leafy Australian cities are notably “birdy”). He could also have noted the house bunting, the ubiquitous street bird alongside house sparrows in North African towns, and the rufous-collared sparrow that plays the same role in Latin American cities. Crows are a special case, and Marzluff has written a thoughtful and informative book about them: In the Company of Crows and Ravens (2005).
As might be expected of these intelligent birds, some of which can figure out how to make and use tools, many species have learned that cities are “Edens for crows.” Nobody shoots them, and there is lots of food. Mark Twain wrote of being pestered by house crows when he visited India in 1896. This species has since spread to East African and Southeast Asian cities as well. American crows, once extremely shy because of widespread slaughter in the countryside, have become confident city-dwellers in the last fifty years. A little later the closely related carrion crow moved into downtown Paris, London, and other European cities. Jungle crows have became so numerous and aggressive in Tokyo that the city has taken measures to control them.
After the exploiters come the adapters. Animal evolution responds powerfully to the new opportunities offered by the suburbs. Evolution, Marzluff reminds us, is not something that happened ages ago. It is in constant motion, as revealed by the almost immediate appearance and spread of strains of bacteria resistant to every new antibiotic that we develop. To illustrate the persistence and speed of animal evolution, Marzluff evokes the famous case of the peppered moth, a species that survives by passing unobserved against the bark of a tree. As tree bark grew darker in sooty Victorian London, paler peppered moths were quickly spotted and snapped up by predators.
The greater survival of the “fitter” (i.e., darker) peppered moths turned the whole population darker. Then the whole population turned paler again when London became cleaner in the twentieth century. Marzluff cites some studies of house sparrows that turned darker in industrial environments, and notes that Italian and Spanish populations, isolated by mountains from those in northern Europe, have evolved into separate species. He speculates that the urban populations of some species may become so biologically distinct over time that they too will form separate species, though this is a notably bold prediction.
Marzluff is at his most interesting when he reminds us that animal evolution is not only physical but behavioral. The birds and animals that adapt to the suburbs learn about feeders and nest boxes and domestic animals. They must above all become more tolerant of human proximity. There are notable differences in behavior between city birds and country birds. In Paris, the city blackbirds (like American robins dipped in ink) run around among the shoppers at the farmers’ market grabbing stray cherries on the sidewalk, while the country blackbirds we see in Burgundy are so timorous that we have only to appear at a window for them to flee the lawn. Marzluff noted in his crow book that rural American crows, expecting to be shot, protest an intruder at the nest with loud caws from a safe distance; urban crows whose nest tree is being climbed remain silent but strike the intruder’s head.
It may be that Marzluff’s own suburb near Seattle, and North American suburbs generally, offer many more opportunities for wildlife than the megacities of Asia and Africa. Marzluff shows also that the gradient works differently in the tropics, where not even the opulent quarters of cities have wildlife as rich as the tropical forest outside. In the tropics, where many species are finely adapted to very precise microhabitats with a highly specialized mix of plants, insects, temperature, moisture, and other conditions, it seems likely that the vast majority of wild bird species are “avoiders.” “Subirdia” may be a more localized and culturally conditioned phenomenon of temperate North America and Europe than Marzluff seems to be saying.
There is no reason to feel complacent that the comparative biological richness of temperate-zone suburbia means that we don’t have to worry about the survival of biodiversity. Only some birds and other animals remain in “subirdia” when the ground is cleared for a new subdivision. The survivors are the more adaptable ones, the ones that already lived in heterogeneous environments and had varied diets and were able to seize opportunities. It may well be that more species of birds are “adapters” than is the case with mammals, amphibians, and arthropods. We have already mentioned the disappearance of most large mammals in “subirdia.” The large beetles that help dispose of dead wood and animal corpses, and amphibians that need access to both land and clean water, have also survived much less well in the suburbs than birds.
Increasingly the wildlife that thrives in “subirdia” may consist of exotic species from elsewhere that can crowd out native ones. If Marzluff worked in Miami instead of Seattle, he would have had to note the dozens of species of parrots present in southern Florida, escaped from the pet trade and in some cases reproducing in city parks. He might have said more about the arrival of alien plant “invasives” such as kudzu, Japanese knotweed, mile-a-minute weed, Oriental bittersweet, and many others that, in the absence of their normal predators, cover whole tracts of ground in the American suburbs and stifle all other forms of life under their impenetrable foliage.
Another problem with “subirdia” is that the “exploiters” can become pests. As Jim Sterba recently showed in Nature Wars: The Incredible Story of How Wildlife Comebacks Turned Backyards into Battlegrounds,* the two-way movement of human settlement into the wild and of adaptable wildlife back into town can produce dismaying interactions as well as pleasing ones. Marzluff is not blind to the downside of the capacity of some natural creatures to coexist with us. He notes that the density of black widow spiders in Phoenix, Arizona, is thirty times that of the surrounding open desert. And he admits that if his house were occupied by that most poisonous of snakes, the black mamba, he would have to get rid of the snake.
For Marzluff, however, the proximity of wildlife is something almost wholly positive, and he feels that flickers drilling holes in his siding and wrens nesting in his mailbox are a small enough price to pay for the pleasure of their company. He understands that adaptation of wildlife to the suburbs is no guarantee of the survival of biodiversity in general but preserves only the adaptable species. The twenty-first century, the “anthropocene” era, will still be a time of mass extinction simply because there are so many “avoiders” in the natural world, especially in the tropics. It may be that “subirdia” is simply the best we can do in the face of the “urban tsunami.”
The richness of animal life in “subirdia” is not automatic, and its human inhabitants can make it richer still by being tolerant of some animal neighbors and encouraging others by applying a little ecological knowledge. Marzluff ends his book on a didactic note by providing ten commandments for enhancing ecological richness in “subirdia.” They are worth repeating in full, but they will not be every American’s cup of tea.
Thou shalt not covet thy neighbor’s lawn. Marzluff enthusiastically supports natural ground cover instead of the ubiquitous sterile lawn, but one may have to fight one’s neighbors and the zoning board in order to carpet a lawn with native plants.
Keep your cat indoors. Marzluff has some good practical suggestions about how to make your cat tolerate it.
Make your windows more visible to birds.
Do not light the night sky. This commandment, like the previous one, may apply to office and apartment towers more than to residences, and that makes it harder to bring the changes about.
Provide food and nest boxes. But you will have to clean them, and expect some nuisance (see Sterba), and realize that they probably do more for your pleasure than for animal survival except in the harshest winter weather.
Do not kill native predators. Even when the Cooper’s hawk is eating the chickadees at your feeding tray.
Foster diversity of habitats within cities and the natural distinction among cities. Note that most of Central Park is emphatically not “birdier” than Yellowstone, but only the parts like “the Ramble” where the ground is kept densely covered with native vegetation despite powerful counterforces—dog runners, park managers enamored of the mowing machine, and horticulturalists with their standard list of exotic plants.
Create safe passage across roads and highways. This is actually being done, more in Europe than here.
Ensure functional connections between land and water. Still further education of city planners, and golf course managers, will be required, as well as some expense.
Enjoy and bond with nature where you live and work. Marzluff has done this, and it has given him contagious joy that shows in the pages of this enjoyable and informative book.