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The Wing of the Swallow

Ben Goldfarb and cover of Crossings

Introduction to Crossings, How Road Ecology is Reshaping the Future of Our Planet by Ben Goldfarb
Posted with permission of the author

If you’ve ever driven across the United States of America, you have passed beneath the wings of a plucky songbird—​smaller than your palm, light as your pocket change, feathered in jaunty blue and umber—​called the cliff swallow. Where other animals flee the human footprint, cliff swallows shelter in its tread. Petrochelidon pyrrhonota should properly be called the bridge swallow, for our steel spans have furnished it with more nesting sites than bluffs and canyons ever did. Once a bird of the western mountains, in the last century cliff swallows have spread onto the Great Plains and across them, plastering their gourd-​shaped mud nests to girders and trusses, feats of avian engineering no less impressive than our own viaducts.

“Once the environment is ruined,” a biologist named Charles Brown told me, “all we’ll have left is rats, cockroaches, and cliff swallows.”

Cliff swallows are gregarious birds whose colonies can number in the thousands. Like most civilizations, theirs are messy: they steal nests, bully others into mating, fight so viciously that they sometimes tumble into rivers and drown. For the last four decades Brown has paid annual visits to more than two hundred nesting sites in Nebraska, trying to figure out what makes swallow societies thrive or fail. He has studied how well they catch insects, how they spread diseases, how they fend off snakes. He has captured more than four hundred thousand swallows in nets and encircled more than two hundred thousand slender legs with coded metal bracelets. Mostly what he has done is drive, more miles than he can hazard, from bridge to bridge, colony to colony. “Ninety-​eight percent of cliff swallows in western Nebraska,” Brown said, “are going to be within fifty feet of a road.”

Near a road, of course, is the most dangerous place an animal can live, and swallows, for all their agility, occasionally fall victim to passing cars and trucks. When Brown and his longtime collaborator, the ornithologist Mary Bomberger Brown, began studying swallows in the 1980s, they picked up these casualties, wings broken and heads crushed, and brought them to their lab. They dressed the bodies—​replacing eyes and viscera with cotton, lacing up feathered breasts like shoes—​and tucked them in a drawer. They didn’t have any plan for the birds; it just seemed proper. The waste of death diminished by the salvage of data.

Years passed. The number of publications mounted; the swallows flourished. In 2012 a new assistant asked Brown to teach him the art of dressing birds. Brown promised there would be plenty of roadkill on which to practice. When summer arrived, though, there was virtually no roadkill to be found—​just empty asphalt and swallows, gloriously alive, taunting the vehicles below.

The epiphany hit Brown like an eighteen-​wheeler: this was not a single-​summer fluke. Swallow roadkill had been dwindling for years. He’d collected twenty dead birds in 1984, when the project began, and twenty more in 1985 and 1986. Then the trend line slanted downward, straight as a ski slope: fifteen in 1989, thirteen in 1991, eight in 2002. By 2011 the toll had dropped to four.

Brown considered various explanations, then dismissed them. There weren’t fewer swallows to be whacked or more vultures bearing away carcasses, nor was there less traveling on his part. No, Brown thought: somehow swallows had become harder to kill.

He found his answer in the corpses themselves. When he stretched a tape measure from the birds’ shoulders to their outermost feathers, he found that car-​struck swallows had longer wings than the average bird he snared in his nets. The difference was slight, no more than a few millimeters, but the gap had unmistakably grown over the years. Brown immediately understood the significance. Long wings were good for straight, lengthy flights: between nests and feeding grounds, for example. Shorter wings were better for maneuverability, for performing the tight pivots and rolls with which a swallow would evade a falcon—​or a flatbed hauling a load of lumber to Omaha. Traffic was weeding clumsier, long-​winged swallows from the population and favoring their nimbler, short-​winged flockmates. It was Darwinian selection in action, so clean and rapid it belonged in a textbook.

“Morphologically, these are not the same birds anymore,” Brown said. But it seemed to me they were also different in some deeper, more metaphysical sense. Centuries ago, before we paved North America, cliff swallows had existed largely beyond human influence; now they were so enmeshed in our world that our infrastructure had infiltrated their DNA. Cliff swallows were a success story, rare beneficiaries of concrete and steel. Yet their triumph had come at a cost—​to the long-​winged martyrs culled from the population and to the birds’ altered genes themselves. Cliff swallows had survived, but as a changed thing. They had been shaped, subtly but intimately, by the road.


When alien archaeologists exhume the rubble of human civilization, they may conclude that our raison d’être was building roads. Some forty million miles of roadways encircle the earth, from the continent-​spanning Pan-​American Highway to the hundred thousand miles of illegal logging routes that filigree the Amazon. Our planet is burdened by three thousand tons of infrastructure for every human, nearly a third of an Eiffel Tower per person. Roads predate the wheel: Mesopotamian builders began laying mud-​brick paths in 4000 bce, centuries before anyone thought to drop a chariot onto a couple of potter’s disks. Today it’s impossible to imagine life without the asphalt arteries that connect goods with markets, employees with jobs, families with each other. “Everything in life is somewhere else,” wrote E. B. White, “and you get there in a car.”

Roads are both logistical essentials and cultural artifacts. They epitomize freedom—​the “architecture of our restlessness,” per Rebecca Solnit, the “two lanes [that] take us anywhere,” per Bruce Springsteen. To us, roads signify connection and escape; to other life-​forms, they spell death and division. Sometime during the twentieth century, scientists have written, roadkill surpassed hunting as “the leading direct human cause of vertebrate mortality on land.” Name your environmental ill—​dams, poaching, megafires—​and consider that roads kill more creatures with less fanfare than any of them. (More birds die on American roads every week than were slain by the Deepwater Horizon oil spill, with the road deaths accompanied by a fraction of the hand-​wringing.) And it’s only getting worse as traffic swells. A half-​century ago, just 3 percent of land-​dwelling mammals met their end on a road; by 2017 the toll had quadrupled. It has never been more dangerous to set paw, hoof, or scaly belly on the highway.

Roads distort the planet in other, more insidious ways. No sooner was Rome’s Via Cassia completed around 100 bce than its surface began to shed sediment into Lago di Monterosi, spawning algal blooms that permanently distorted the lake’s ecosystem. Phytophthora lateralis, an invasive fungus that attacks cedar trees, hitchhikes in the patterns of truck tires. The little red fire ant, a merciless insect notorious for stinging the eyes of elephants, has exploited logging tracks to spread through Gabon sixty times faster than it would have otherwise. Pavement itself blankets less than 1 percent of the United States, yet its influence—​the “road-​effect zone,” to use ecological jargon—​covers a full 20 percent. Park your car on the shoulder and bushwhack half a mile into the woods, and you’ll still see fewer birds than you would in an unroaded wilderness. Hike two miles more, and you’ll still see fewer mammals. If you’re a Kerouac reader, you grew up steeped in the dogma that highways represent freedom. If you’re a grizzly bear, they might as well be prison walls.

The repercussions of roads are so complex that it’s hard to pinpoint where they end. British Columbia’s caribou herds have dwindled to furtive bands, in part because logging and mining roads have permitted the ingress of wolves—​a human-​caused disaster disguised as natural predation. Nearly a fifth of America’s greenhouse gas emissions are coughed out by cars and trucks, and the transportation sector is the fastest-​growing contributor to climate change; meanwhile, the rise of electric vehicles, whose batteries depend on lithium and other metals, has catalyzed a mining boom that threatens to disfigure landscapes in places as disparate as Chile, Zimbabwe, and Nevada. Even habitat loss, the most thorough eraser of wildlife, is a road problem. Before you can log Alaska’s rainforests or convert Bornean jungles into oil-​palm monocultures, you need roads to transport the machinery in and the product out. Roads are, you might say, the routes of all evil.

Yet roads select winners as well as losers. Arizona’s highways funnel rainfall into ditches and thus soften desert soils for pocket gophers, whose tunnels parallel the shoulder like subway lines. Vultures, ravens, and other cunning scavengers are ascendant, their diets subsidized by roadkill. Butterflies whose prairies have been devoured by cornfields find succor in unkempt strips of roadside milkweed. In Britain such habitat is called the soft estate—​a suggestion that roads are capable of creating new ecosystems, even as they shatter existing ones. A biologist once led me beneath a highway bridge to show me hundreds of little brown bats roosting in its crevices, seemingly unbothered by the traffic thumping overhead.

Considering the outsized effects of roads, it’s perhaps surprising that they didn’t truly receive their scientific due until the late twentieth century. One afternoon in 1993 a landscape ecologist named Richard Forman was standing in his Harvard office with a few students, admiring a satellite photograph of a forest. Forman was expounding on the forest’s features—​where the water flowed, why people had put houses where they had, how the animals moved through it—​when he paused. “I noticed the long slice going diagonally across the image,” he recalled to me. “It was a two-​lane road through the forest. I said, gee, we know a lot about the ecology of everything else in this image, but we don’t know much about the ecology of that.” Inspired by inattention, Forman soon coined an English term: road ecology, defined loosely as the study of how “life change[s] for plants and animals with a road and traffic nearby.”[*]

He did not immediately attract disciples. When a major government committee invited Forman to present his new field to transportation higher-​ups the next year, he was met with polite laughter. “You’re not here to make us stop running over animals, are you?” one engineer asked, cocking an eyebrow. As the 1990s wore on, though, road ecology gained steam. Forman and other pioneers published papers, wrote textbooks, held conferences that lured curious officials. “All of a sudden,” Forman said, “it became mainstream.”


My own introduction to road ecology came in 2013, the year I embarked on a trip across the continent to write about an extraordinary scheme called the Yellowstone to Yukon Conservation Initiative. The goal of Yellowstone to Yukon, or Y2Y, is boggling: its advocates envision a network of connected habitats that would permit animals to wander unhindered along the spine of the Rockies, a region that spans five American states and four Canadian provinces and territories. Such a corridor would preserve migration routes for elk and caribou, permit far-​ranging creatures such as wolves to mingle and mate, and help sensitive animals like wolverines flee northward as climate change nips at their heels. The initiative’s emblem is the grizzly, whose expansive requirements make it a useful proxy for other forms of life. An ecosystem that can support bears is probably healthy enough for everyone else.

To the uninitiated, it sounded far-​fetched. Soon after Y2Y’s inception, The West Wing parodied it as the “Wolves Only Roadway,” the vanity project of humorless tree huggers who get laughed out of the White House. But the show’s writers, like most of Y2Y’s critics, misunderstood the concept. Y2Y wasn’t a discrete pathway; it was a continental jigsaw riddled with missing pieces, most of them at the fragile margins where wildlands and settlements collided. The mission of Y2Y and its many partners was to plug those holes, to help bears and other animals safely navigate the Rockies without running afoul of humans. In British Columbia, I toured protected grainfields that grizzlies used to commute between mountain ranges at night. In Montana, I sniffed offal in an electric-​fenced paddock where ranchers were composting their dead cows rather than permitting them to fester in bear-​enticing boneyards. (Few travelers, human or ursine, can resist fast food.)

Yet Y2Y’s deepest cuts remained mostly unhealed. The region was riven by enough numbered roads to fill a sudoku puzzle: I-​90 and Highway 3 and Highway 20, routes 95 and 40 and 12 and 212, spiderwebbed otherwise wild lands. I drove highways that ended lives—​I lost track of how many elk littered the shoulder on Crowsnest Pass—​and others that cleaved grizzly populations into lonely clusters. Roads, I began to realize, were not merely a symptom of civilization but a distinct disease.

Among the roads within the Y2Y corridor’s ambit was U.S. 93, which traverses Montana on its 1,300-​mile jaunt from Arizona to the Canadian border. Like so many highways, U.S. 93 had been built heedlessly in the 1950s, plowing through wetlands, elk meadows, and a vast reservation belonging to the Confederated Salish and Kootenai Tribes. When, in the 1990s, state and federal agencies sought to expand U.S. 93 from two lanes to four, tribal officials demanded the chance to provide input on the reconstruction. A wider, faster road might be safer for drivers, but it would also slaughter more deer, elk, bears, and other animals foundational to the tribes’ culture. “The road is a visitor,” the tribes insisted, that should “respond to and be respectful of the land and the Spirit of Place.”

The Salish and Kootenai flexed their legal and moral muscles, and, when U.S. 93 was finally reconstructed, engineers included around forty wildlife crossings—​a network of underpasses, tunnels, and culverts that allowed animals to slink beneath the highway unimpeded. Roadside fencing kept creatures off the highway and guided them toward the passages. The project’s flagship structure was an elegant bridge designed principally for that avatar of wildness, the grizzly bear. In aerial photos, the overpass looked at once futuristic and anachronistic, a green parabola that vaulted over the highway with Middle Earthish grace. If roads were a disease, wildlife crossings seemed like a treatment.

That October, I drove U.S. 93 in the company of Marcel Huijser, a lean, grizzled road ecologist who had begun studying the highway back when the crossings were still in their planning stages. I was then more or less ignorant of road ecology, let alone how one became a road ecologist, so, as we headed north from Missoula, I asked Huijser to tell me about his past. He’d grown up in the Netherlands, a country that packs one of the world’s densest road networks into a landmass one-​ninth the size of Montana. Bears and wolves had long since fled the overbuilt Dutch landscape, so Huijser had studied hedgehogs, which popped in and out of gardens like cheerful neighbors. “Everybody thinks they’re cute and wonderful,” Huijser said as we rolled past golden cottonwood galleries. “They’re a very sympathetic animal. People want to make things pleasant for hedgehogs.”

Alas, hedgehogs—​small, plodding, nocturnal—​were practically designed to be roadkill, and Huijser’s calculations suggested that hundreds of thousands were being crushed each year. This was a familiar story in the Netherlands. Roads, dikes, canals, and towns had broken the country’s landscape into pieces, leaving little space for hedgehogs and other fauna. In 1990 the country, with typical Dutch ingenuity, had launched a national defragmentation plan that ultimately led to the construction of more than eight hundred wildlife crossings on national highways, from badger pipes to deer bridges. Huijser’s research had shown that hedgehogs prefer the ecotones where forests and grasslands meet, helping planners situate new passages. “One of my statements during my PhD defense was that they’re really edgehogs,” he said, sheepish at the pun.

In 1998, Huijser met his wife, an American conservationist named Bethanie Walder, at—​what else?—a road conference. He eventually relocated to Montana to work for the Western Transportation Institute, the research team tasked with studying the U.S. 93 crossings. In the years that followed, Huijser and his colleagues scooped up deer pellets, pored over photos snapped by motion-​activated cameras, and crouched to inspect hoof- and pawprints deposited in sandy soil. By the time I visited, animal collisions had fallen by around three-​quarters, and Huijser’s team had documented tens of thousands of successful traversals through the crossings: coyotes, foxes, bobcats, elk, otters, porcupines, moose, grizzlies. Highway 93, Huijser told me, “compares favorably with anywhere else in the world in the number and density of structures”—​even the Netherlands.

As Huijser parked beside the overpass, dusk descended. He unlocked a gate in the roadside fence, and we slipped through as though into a portal, passing from the world of the highway to a wild parallel dimension. We climbed the gentle slope, autumn grass crispy beneath our boots. I reached the crossing’s apex and peered down at the muffled stream of Missoula-​bound traffic. The horizon glowed orange. The evening was sharp with oncoming winter. Though we were just three stories above the earth, I felt buoyant. The air shimmered with supernatural possibility; at any moment, I thought, a grizzly would emerge from the pines and trudge onto the bridge.

Huijser didn’t share my muted awe. He paced the crossing and pointed out design flaws. He was displeased with the bridge’s spartan landscaping: a few carefully positioned brush piles, he suspected, would aid mice and voles. And its sightlines were too exposed. His cameras had recently captured a black bear fleeing the headlights of an approaching car. “A visual screen would be helpful,” he said. Shrubbery or an earthen berm might do the trick. “It could just be a wooden fence.”

Huijser, it occurred to me, was attempting to inhabit other beings’ Umwelt, their subjective lived experience. Road ecology was an act of interspecies imagination, a field whose radical premise asserted that it was possible to perceive our built world through nonhuman eyes. How does a moose comprehend traffic? What sort of tunnel appeals to a mink? Why do grizzly bears prefer crossing over highways while black bears go under? These questions had empirical answers, but they also required ecologists to think like wild animals—​empathy manifested as science.

To us, roads are so mundane they’re practically invisible; to wildlife, they’re utterly alien. Other species perceive the world through senses we cannot fathom and experience stressors and enticements we hardly register. Bats are lured astray by streetlights, snails desiccate as they slog across deserts of asphalt, and seabirds crash-​land on the shiny tarmac they mistake for the ocean. Consider the sensory experience of, say, a fox approaching a highway: the eerie linear clearing that gashes the landscape, the acrid stench of tar and blood, the blinding lights staring from the faces of thundering predators. When Hazel, the rabbit protagonist of Watership Down, encounters his first road, he confuses it for a river, “black, smooth and straight between its banks.” A passing car “fill[s] the whole world with noise and fear.” “Now that I’ve learnt about it,” he adds, “I want to get away from it as soon as I can.”


Road ecology inverts our oldest joke about animals and transportation: Why did the chicken cross the road? Embedded in that chestnut is an assumption—​that the road is inviolable and eternal, as fixed in its course as a river. The road is a given; it’s the fowl whose actions demand explanation. But the riddle’s logic is backward. It’s the animals who have always moved, the road that’s the upstart. A better question might be, Why did the road cross the land?

This framing isn’t always comfortable. When we don’t ignore roads, we dismiss their toll as the inevitable cost of modernity. Other forms of human-​caused animal death are deliberate: we pull the trigger, set the trap, order the cheeseburger. But few among us ever flatten an animal on purpose. Like most people, I at once cherish animals and think nothing of piloting a three-​thousand-​pound death machine. The allure of the car is so strong that it has persuaded Americans to treat forty thousand human lives as expendable each year; what chance does wildlife have? One summer, in Alaska, I hit a feisty songbird called a yellow-​rumped warbler—​a death I didn’t discover until I found the delicate splash of feathers wedged in the grille the next day. I’d killed not with malice but with mobility. “We treat the attrition of lives on the road like the attrition of lives in war,” the writer Barry Lopez lamented. “Horrifying, unavoidable, justified.”

This is particularly true in the United States, home to the world’s longest road network, at four million miles. Our mid-​century automotive revolution spawned not only highways but also parking lots, driveways, suburbs, pipelines, gas stations, car washes, drive-​throughs, tire shops, and strip malls—​a totalizing ecosystem engineered for its dominant organism, the car. For all its grandeur, though, America’s highway network is relatively static. Although we spend almost $200 billion on our roads annually, most goes toward repair rather than new construction. Granted, American wildlands are hardly safe from ill-​conceived development: Florida, for one, has been scheming up new toll roads in panther habitat, and even routine highway maintenance projects have an uncanny knack for adding lanes and worsening traffic. Even so, our country’s asphalt limbs have mostly ceased to elongate, petrified into something like their eternal shape.

Instead, we’re exporting our autocentric lifestyle. More than twenty-​five million miles of new road lanes will be built worldwide by 2050, many through the world’s remaining intact habitats, a concrete wave that the ecologist William Laurance has described as an “infrastructure tsunami.” Astoundingly, as of 2016, three-​quarters of the infrastructure that will exist by the middle of this century had yet to be built. Although it’s easy to denounce the tsunami, I benefit from roads as much as anyone: I eat avocados trucked from California; I get pizza delivered to my doorstep; I rely on America’s marvel of a highway system to reach friends and hospitals and airports. (And I confess to feeling what one Volkswagen ad campaign called Fahrvergnügen, the pleasure of driving.) Roads pose the same queasy conundrum as climate change: having profited wildly from growth, can wealthy nations deny less-​developed countries the benefits of connectivity?

Road ecology offers one path through this thicket. North America and Europe constructed their road networks with little regard for how they would affect nature and even less comprehension of how to blunt those effects. Today, in theory, we know better. Road ecology has revealed the perils of reckless development and pointed us toward solutions. Over the last several decades, its practitioners have constructed bridges for bears, tunnels for turtles, rope webs that allow howler monkeys to swing over highways without descending to the forest floor. On Christmas Island, red crabs clamber over a steel span during their beachward migrations; in Kenya, elephants lumber beneath highways and railroads via passages as tall as two-​story houses. And road ecology has yielded more than crossings: we’ve also learned to map and protect the migrations of cryptic animals, to design roadsides that nourish bees and butterflies, and to deconstruct the derelict logging tracks that lace our forests—​proof that old mistakes need not be permanent.

Quoth the late-​night sage John Oliver, “Infrastructure isn’t sexy.” Clearly he hasn’t talked to a road ecologist. Roads have become one of conservation’s most urgent topics, the focus of hundreds of scientists operating in dozens of countries. Over several years I traveled the world meeting some of them: the biologists tracking anteaters across Brazilian highways, the conservationists building bridges for California’s mountain lions, the animal rehabbers caring for Tasmania’s car-​orphaned wallabies. While Crossings is rife with other species—​mule deer and capybaras, wombats and monarch butterflies—​it also considers how our own lives have been captured by pavement and how we can reclaim them. Wild animals, the naturalist Henry Beston wrote, are neither our brethren nor our underlings; instead, they are “other nations, caught with ourselves in the net of life and time.” And the road ensnares us both.

This book is about how we escape.

[*] As European road ecologists are quick to remind their American colleagues, “road ecology” was actually a translation of straßenökologie, a neologism coined by a German scientist named Heinz Ellenberg in 1981.

Watch Ben Goldfarb's recent presentation on Crossings here. You can register and then view the enlightening program any time.