Last year a wildlife biologist spent four days bird-watching in Yellowstone National Park, one of America’s most pristine wild lands. By the time he left, he had tallied 26 species. Soon after, he traveled to New York’s Central Park. In two days he counted 31.
There was nothing scientific about either trip: just a man and his binoculars, looking for birds. But the findings contradicted what he had learned as a conservation biologist: that cities—destructive, disruptive, and, for some species, lethal—were bad for wildlife. So how could it be that Central Park, as well as neighborhoods in other cities he and his fellow scientists had studied in recent years, boasted such an impressive array of birds?
John Marzluff, the scientist, is well known for his research on, among other topics, the intelligence of crows and ravens. In his new book, Welcome to Subirdia: Sharing Our Neighborhoods With Wrens, Robins, Woodpeckers, and Other Wildlife (Yale University Press), Marzluff examines the effects of urbanization on a variety of birds.
In more than a decade of research in and around Seattle, where he is a professor of wildlife science at the University of Washington, Marzluff and a small army of graduate students discovered a consistent pattern: Bird diversity grew from the city center, peaked in the suburbs, and dropped again in the forested areas between Seattle and the Cascades.
“We had discovered subirdia,” Marzluff writes. “Now I was really perplexed.”
Sprawl, after all, has contributed to the demise of many creatures. And climate change threatens, too: Scientists with the National Audubon Society warned this month that warmer temperatures will force nearly half of North American bird species to live in smaller ranges within decades; those that can’t adapt may go extinct. Yet subirdia—"the confluence between city and country that promotes a mutual exchange of plants and animals,” as Marzluff defines it—is clearly an avian hub.
Researchers studying other species of wildlife have been similarly intrigued by these not-so-wild places. Until recently, many scientists and scholars regarded humans and urban areas as the antithesis of nature. But as urban ecology has matured, an interdisciplinary web of researchers including biologists, anthropologists, sociologists, economists, and others are training an ever-inquisitive gaze on the soil, water, plants, insects, and animals of cities and suburbs. At the Ecological Society of America’s annual meeting last month, for instance, dozens of presentations explored urban questions: tree communities in Portland, Ore.; spiders in Central California city gardens; migratory birds who rest on a third of an acre of parkland in Newark, N.J.
For some species, urban and suburban development is traumatic and costly, as disruptive as any eviction notice. But others thrive.
Glenn Bartley, Vireo
For many birds, the suburbs, as Marzluff explains, afford a wide variety of habitats. The trees, flowers, shrubs, ponds, and bird feeders that dot our neighborhoods make them attractive to many species. Add the golf courses, office parks, and retention ponds that are hallmarks of many suburban landscapes, and subirdia becomes downright appealing.
To explain why some species flourish and others decline in the face of rapid environmental changes, Marzluff invokes a trio of terms coined by Robert Blair, a conservation biologist at the University of Minnesota-Twin Cities: avoiders, adapters, and exploiters.
Avoiders, like the red-eyed vireo and the wood thrush, are sensitive species that flee the pavement, abundant lights, pollution, and noise of urban and suburban areas. Adapters, meanwhile, adjust to the changing landscapes of cities and towns that approximate their native habitats. “If they were plants, we’d call them weeds,” Marzluff writes, noting that these birds—finches, other thrushes, sparrows, and crows among them—live fast, reproduce often, and die young.
Exploiters do just fine around humans and the structures we build; in fact, they rarely exist in places without people. Among them are the familiar “fab five,” as Marzluff calls them, present in nearly every city scientists have studied: the Canada goose, European starling, house sparrow, mallard, and rock pigeon.
But how or when do certain avoiders decline? When, and in what order, do adapters or exploiters appear? Are the gains and losses sudden or gradual? These are among the questions Marzluff and his students set out to answer.
Seattle, they noticed, was changing and developing at a rapid pace, so in addition to established neighborhoods, they examined forests slated for residential development. The researchers employed two methods. The first is a common tool in wildlife monitoring known as a point count, used to document changes in bird populations. Every summer for 12 years, researchers visited 26 designated points across suburban Seattle; over the course of the study, they visited more than 100 sites at least once. At each point, researchers stood quietly, watching and listening for as many birds as they could detect.
Spot mapping, the second technique, provides additional context through close observation of individual birds over the course of their lives: Do the birds live nearby, or are they flitting in to partake of birdseed at backyard feeders? Are their populations sustainable? How long do they live?
Disguised in some cases as construction workers or prospective home-buyers, researchers poked around backyard bushes and street corners at odd hours to catch and color-band birds, and to find their nests. (Neighbors, police officers, and contractors were, at best, curious.) By 2010, the team had tallied more than 55,000 individual birds, representing 111 species. They concluded that the fab five, along with the American crow and the house finch, did fine. Adapters like song sparrows and spotted towhees fared well, too, as residential development turned forests into clearings.
But for a handful of the species Marzluff and his students tracked, like the tiny Pacific wren and the Swainson’s thrush, life in subirdia appeared to trigger a nomadic, and ultimately unsuccessful, search for a suitable home. By the end of the study, Marzluff had deemed the Pacific wren—"the icon of unsettled northwestern forests"—one of subirdia’s clear casualties.
As a graduate student in the 1980s, Marzluff did a study of pinyon jays in Flagstaff, Ariz. When he and his fellow researchers sent out the paper for review, he recalls in a recent interview, they received the same caveat each time: “This is a town,” the reviewers reminded them. “Everything you’re seeing is different.”
In those days, “real” nature meant wilderness, unsullied by human hands. Biologists sought pristine locations for their research. But during the 1990s, that thinking began to shift. Cities and suburbs offered unique ecosystems of their own.
Many of the ecological patterns and processes of urban landscapes were similar to those that occur in a natural scenario—competition, predation, decomposition. But there were unique aspects to urban environments, too, that piqued scientists’ curiosity, says Nancy E. McIntyre, professor of biological sciences and curator of birds at Texas Tech University’s Natural Science Research Laboratory. Some stressors and selective forces at work in cities were different than those in forested or “natural” areas, she explains.
In the late 1990s, the National Science Foundation began funding ecosystem studies in Baltimore and Phoenix as part of its Long-Term Ecological Research Network. (Most of the network’s other two dozen or so sites, some of which have been under way since the early 1980s, are not located in urban areas.) Drawing on the expertise of researchers in the biological, physical, and social sciences, the studies aim to track the two cities’ ecosystems as their metropolitan areas grow and change.
The Baltimore research, led by Steward T.A. Pickett of the Cary Institute of Ecosystem Studies, includes a bird-monitoring project that has continued for more than a decade. In gritty and tony neighborhoods, vacant lots, and city parks, field technicians spend every summer tracking bird populations at 132 sites. Researchers visit each site and take up the point-count method also favored by Marzluff, using a stopwatch as they look and listen for birds.
Ela-Sita Carpenter, an aspiring wildlife biologist and native Baltimorean, was the bird-monitoring project’s field crew leader this summer. On a recent morning, she’s standing on a gravel road in a forested swath of parkland in West Baltimore. It’s one of the most verdant points in the study’s constellation of sites.
A typical outing to this spot might yield sightings of American robins, American goldfinches, house finches, chimney swifts, and gray catbirds. On this day, an eastern wood pewee—by Marzluff’s definition, an avoider that thrives only in wooded areas—trills its eponymous song, and a Carolina chickadee scolds from a nearby branch. Carpenter has heard a great horned owl here, hooting at dusk. Once she spotted a common nighthawk.
By design, the study oversamples the city’s neighborhoods, to better understand how changes in residential areas affect birds, says Charles Nilon, a professor of fisheries and wildlife at the University of Missouri at Columbia, who oversees the Baltimore study’s bird monitoring. Researchers have found that Baltimore is indeed home to an astonishing array of birds. In the city alone, they’ve detected one-third to one-half of the 130 species the Baltimore Bird Club has tallied in a region that also includes suburban Baltimore County.
But the study includes a handful of points in industrial areas, too. One lies southwest of downtown, not far from the Fort McHenry Channel, just outside the locked gates of a storage facility for refined petroleum. It seems like a place that even the most intrepid adapters or exploiters would find hard going: Aside from a couple of trees and shrubs, the area is all pavement and barbed wire, rutted roads ferrying trucks to and from nearby waste-water-treatment plants, rail yards, and asphalt manufacturers. But birds do live here: Since 2005, researchers have documented 23 species, including the killdeer, purple finch, and eastern kingbird.
On this day, no birds were visible. But as we headed back downtown, looping along a vertiginous cloverleaf over the middle branch of the Patapsco River, a dark speck appeared in the hazy sky. An osprey, wings beating, soared above the interstate and the hum of Baltimore’s urban habitat far below.
Madhusudan V. Katti has a metaphor for cities: coral reefs. Mollusks build them, presumably for their own purposes. Yet they are vast, complex structures, home to flora and fauna of astonishing diversity. The human metropolis is no different, says Katti, an associate professor of biology at California State University at Fresno. “Why do we think of cities as artificial just because humans built them?”
For an evolutionary ecologist, he adds, urban areas also offer a rare opportunity. Hurricanes or volcanoes are powerful disturbances that can bring about major changes in an ecosystem. So is development. But there’s a difference: “I can’t interview the hurricane,” Katti says, “but I can talk to people.”
Indeed, as Katti and others peel back layer after layer of scientific inquiry into urban environments, they seem keen to move beyond the mere documentation of species to qualitative questions about animals’ health and reproductive patterns. Birds are particularly revealing because of their close interactions with humans, says Paige S. Warren, an associate professor of environmental conservation at the University of Massachusetts at Amherst who, with Nilon, oversees the bird component of the Baltimore study.
“Cities can be hot spots of innovation for people,” Warren says. “They’re also places of dense agglomerations of poverty. I think we see the same thing in wildlife, but we haven’t investigated it.”
Birds aren’t the only animals that live in cities, of course. Raccoons, foxes, coyotes, mountain lions, even bears can be city dwellers, too. As Marzluff says in a chapter titled “Beyond Birds,” feathered creatures’ ability to fly gives them a distinct advantage when it comes to surviving the ups and downs of urban and suburban living. Life is a lot more challenging for animals that must walk, hop, or slither to negotiate an urban landscape.
Still, many of the same questions still apply. Stephen DeStefano, a colleague of Warren’s at UMass, studies the influence of urban and suburban development on bears and moose in New England. Bears are thriving there, increasing in numbers and broadening their geographic range every year.
“On the one hand, bears are doing great,” says DeStefano, a research professor at the university and leader of the U.S. Geological Survey’s Massachusetts Cooperative Research Unit. “But we’re starting to wonder, What does this mean for bears in general?” For one thing, their diet centers largely on garbage; many of the bears DeStefano studies have a particular fondness for doughnuts. Is that necessarily good?
Marzluff has been asking similar questions since he was a boy. Growing up in Lawrence, Kan., in the early 1970s, he grew fond of a field and a wooded area near home, treating them, he says, as his own “personal wilderness.” Then the survey stakes went up: The area was slated for residential development, and Marzluff’s father was among the developers. What will happen to the animals? the son asked his father. The animals will move to a new place, came the reply. “I think we both knew the answers weren’t that simple,” Marzluff writes. “The wildlife would either have to adjust to their new human neighbors or perish.”
Forty years later, Marzluff returned to Lawrence. Wandering around the neighborhood that had replaced his beloved woods and field, he found nearly all of the bird species he had recalled as a kid. Gone were the red-headed woodpeckers, a species that has declined across the United States as forested lands are developed. But nuthatches, chickadees, hawks, warblers, and cardinals were abundant. The visit provided a personal perspective on the very changes he had studied for a decade in Seattle. Still, the picture felt incomplete.
“The current assembly of birds seemed OK,” he writes. “But what was really lost or gained over those 40 years?” Only science, he concluded, could provide the answers.
Libby Sander, a former Chronicle reporter, is a freelance writer who lives in Washington, D.C.