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No Way Home

The Decline of the World's Great Animal Migrations

ISLAND PRESS

Empty Skies

PICK THE RIGHT NIGHT and you will hear them. A September night, perhaps, when a northwest wind has swept the clouds from the sky and the stars are out in full force. As midnight approaches, find a quiet spot away from the rumbles and groans of urban life and listen carefully. Soon you will hear soft chirps and whistles drifting down from the sky. These are the calls of migrating songbirds. A thousand feet above you, extending for hundreds of miles in all directions, is a vast highway of little birds—millions of thrushes, warblers, flycatchers, tanagers, vireos, and sparrows—heading south. A few skilled birdwatchers can distinguish the call notes of the different species. A high, clear "chip" marks a northern parula warbler en route to the Caribbean; a sharp buzz identifies a blackpoll warbler headed for the Amazon basin; a gentle whistle denotes a veery traveling from Canada to Colombia. The sky is alive.

The imagination takes flight. What would it be like to travel with the songbirds, to fly in the stillness of the night across the continents, illuminated from above by the stars and from below by the lights of cities? For the time being, the closest one can get to actually flying with the songbirds is to travel with Martin Wikelski. A professor of ecology at Princeton University, Wikelski is determined to figure out what he calls "the rules of migration," the physiological and behavioral cues birds use to decide when to migrate, where to go, and when to stop.

Wikelski's interest in birds dates back to his childhood in Bavaria, where as a teenager he captured and banded birds at a research station under the tutelage of local ornithologists. One spring, he decided to band the barn swallows that were nesting in his village. A year later, he recaptured some of the same birds in the same places. The realization that they had flown all the way to Africa and then returned to this tiny corner of Germany spawned a lifelong interest in migration. Blessed with a keen analytical mind, limitless energy, and an infectious enthusiasm for science, Wikelski is now one of the premier biologists of his generation. Dissatisfied with the idea of studying migratory birds in a laboratory, he instead has chosen to pursue them in the wild, taking advantage of recent breakthroughs in the field of electronics to track their movements.

Each spring for the past five years, Wikelski has journeyed to central Illinois to study migration. Dawn on this particular day finds him stringing a line of nets through the middle of a small woodlot. The fineness of the mesh renders it almost invisible to birds, and before long an itinerant Swainson's thrush, flying through the trees, is ensnared. Wikelski gently extracts the tangled bird, smoothes its feathers, and weighs and measures it. He then attaches a radio transmitter, smaller than a fingernail and lighter than a dime, to the feathers on its back. He next injects the thrush with a tiny amount of doubly labeled water, which has a slightly different chemical signature than ordinary water but is otherwise identical and harmless. He then releases the bird, which darts into a nearby thicket and begins to preen its feathers fastidiously, as though trying to cleanse itself after the indignity of capture. Wikelski and coworkers retreat to the edge of the woodlot where, with the aid of a handheld antenna, they are now able to track the location of "their" thrush.

The bird spends most of the day searching for insect prey on the forest floor. Toward dusk, it appears to settle down for the night in a safe spot on the branch of a small tree. Motionless, eyes closed, it hardly seems like a bird on the go. But shortly before nine o'clock, the thrush stirs. It flies to another branch near the top of a taller tree and looks around. Suddenly, it launches into the night sky, compelled to complete its journey from its winter home in Brazil's lush rain forests to its summer home in the cool spruce forests of Canada. For Wikelski and company, the chase is on.

The scientists pile into an ancient wreck of a station wagon—a 1982 Ford—that has been customized for their research. A hole drilled through the roof allows them to insert a receiving antenna that can be controlled by the driver. The passenger's seat has been converted into a computer docking station, upon which sits a laptop linked to a GPS satellite. Wikelski clamps on a pair of headphones and listens for the signal from the transmitter on the migrating thrush. With his left hand on the steering wheel and his right hand manipulating the receiving antenna, he chases after the bird. The car careens down dark country roads at an alarming speed, the headlights illuminating row after row of young corn, in pursuit of the bird flying above them.

It's not a fair match. This thrush is built for air travel. Its hollow bones keep its body weight down to little more than an ounce. Its tapered wings are well designed to provide lift and forward velocity. As it cruises ahead at a ground speed of thirty-five miles per hour, a system of air sacs connected to its lungs assures a continuous, unidirectional flow of air through the lungs, greatly increasing the efficiency of each breath. Moreover, the bird is equipped with a remarkably sophisticated orientation and navigation system. It has an internal compass, capable of reading the earth's magnetic field, which it sets each day by noting the position of the setting sun. At night, it can determine its position and direction of travel by tracking the apparent movement of the stars, much the way sailors have done for millennia. The thrush, in short, instinctively knows where it wants to go and how to get there. The ornithologists, on the other hand, are restricted to the existing road network, which necessitates frequent midcourse corrections and an average speed well in excess of the thrush's. A colleague in the backseat tracks the thrush's journey on the computer and offers occasional guidance to Wikelski. "It's heading north-northeast now ... turn right at the next intersection ... drive faster, we're losing the signal."

They will drive until they lose track of the bird or until the thrush finishes its journey for the night and lands somewhere to rest. Dawn may find the men anywhere from two hundred to four hundred miles from home, in completely unfamiliar surroundings, searching for the bird. To gain any useful data, they must recapture their thrush and take a small blood sample to determine how much of the doubly labeled water has been metabolized. With this information, the scientists can calculate the energetic costs of migration. Understanding the energetic demands of long-distance migration may prove crucial to determining how many rest or refueling stops the birds require while traveling between their breeding grounds and wintering grounds. Migratory birds fatten up prior to undertaking their long journeys—it's akin to filling the car up with gasoline before a trip. Wikelski wants to figure out how rapidly they burn up that fat while migrating—in essence, how many miles per gallon they get.

A migrating Swainson's thrush typically puts down before dawn, usually in the shelter of some trees or bushes. In the vast agricultural expanses of the Midwest, the only available shelter may be the shrubbery and trees growing in the backyard of a farmhouse. Such is the case on this particular morning, when Wikelski and company find themselves parked outside a house in the middle of nowhere. What follows is arguably the most difficult part of the experiment: summoning the courage to knock on the door of some stranger's house at six in the morning to request permission to set up a line of nets in the backyard for the purposes of capturing a wayward thrush. Needless to say, the ability to charm a suspicious Midwesterner at an ungodly hour of the morning is one of the prerequisites for this sort of work.

There are two great riddles surrounding bird migration: why do birds undertake such long journeys, and how they do it? The "why" riddle is usually explained in terms of opportunism: the birds are taking advantage of the incredible flush of insects produced each summer in the northern latitudes. (In the southern latitudes, a mirror-image migration occurs: some birds that breed in southern South America or southern Africa head north for the austral winter.) But given the challenges associated with flying thousands of miles each year, one cannot help but wonder whether the effort and perils of long-distance migration outweigh the benefits. Wikelski's research provides at least a partial answer. It turns out that the energetic demands of migration are not as great as once believed. An hour of flying is certainly much costlier from an energetic perspective than an hour spent resting in the forest, but only by a factor of four or five (not the twelvefold increase in metabolic demand previously assumed). As a result, the thrushes do not expend that much more energy flying than they would if they simply spent the night huddled in a woodlot in central Illinois. In fact, Wikelski has calculated that actual flight constitutes less than a third of the total energy expenditure of a migrating Swainson's thrush as it makes its way from Brazil to Canada.

During its spring migration, the thrush also benefits from the prevailing winds over the eastern Caribbean and the southeastern United States, which are mostly from the southeast. Having a tailwind for much of its journey increases both the speed and fuel efficiency of the migrating thrush. (In contrast, during the fall the prevailing winds tend to be headwinds, which reduce the birds' speed and increase energy expenditure. Fall migrants, therefore, often must wait several days until a cold front sweeps down from the north and delivers a good tailwind.)

There is little question that long-distance migration has been a very successful strategy for birds. Populations of many of North America's warblers, vireos, thrushes, and other migratory species number in the tens or even hundreds of millions. In the boreal forests of New England or Canada, they may outnumber nonmigratory songbirds by a ratio of ten to one during the breeding season. Yet long-distance migrants typically lay fewer eggs per clutch and raise fewer clutches per year than do nonmigratory birds and short-distance migrants such as robins and cardinals. For this population differential to make any sense mathematically, the long-distance migrants must, on average, outlive the residents, which means they must have a reasonably high rate of survival during migration.

Migration is also a very old strategy among birds. Hesperornis, a genus of flightless diving birds that roamed the seas during the Cretaceous Period some sixty-five million to eighty million years ago, was probably migratory. Its fossil remains have been found in areas that could not have sustained open water year-round, suggesting that it swam to warmer climes during the winter.

Yet the migratory lifestyle, which has been a resounding success for trillions of birds for millions of years, increasingly looks like a losing proposition. Evidence suggests that populations of migratory birds in both the New and Old Worlds have plummeted in recent decades, a decline that carries the potential for significant ecological damage far beyond the loss of the birds themselves.

The evidence for this decline, however, is complex and confusing. In North America, there seems to be a near-unanimous sentiment among birdwatchers over the age of forty or fifty that the phenomenon of migration just isn't what it used to be. Veteran birdwatchers, myself included, look up at the trees and recall a time when the springtime forests rang with the songs of far more warblers, thrushes, vireos, and tanagers than one currently encounters. The trouble with such reminiscences is that they are based on hazy memories rather than hard numbers; their scientific value is negligible. (That so many birdwatchers believe there has been a decline seems noteworthy nonetheless.)

More convincing are long-term census data demonstrating a drop in migratory bird numbers across large portions of the globe. Unfortunately, such data are few and far between. Most nations or states do not have any sort of comprehensive monitoring program for birds, and the few such programs in existence are relatively recent. Within the United States, only a handful of parks and nature preserves have bird census data going back more than a decade or two, but these few, precious sites provide some important clues as to what may be happening to migratory bird populations.

Consider the case of Rock Creek Park, one of Washington, D.C.'s oldest and finest parks. It remains an island of green in a region of sprawling suburbs, ubiquitous shopping malls, and expanding freeways. Since the late 1940s, a succession of dedicated birdwatchers has counted the breeding birds within an eighty-acre section of the park. Their data reveal an ominous drop in populations of songbirds that breed there but then winter in Latin America and the Caribbean. The black-and-white warbler, hooded warbler, and Kentucky warbler no longer nest in the park. Populations of ovenbirds and red-eyed vireos, once two of Rock Creek's most abundant breeding birds, have dropped by approximately 90 percent. In total, the number of migratory songbirds breeding in Rock Creek Park has dropped by 70 percent over the past half century. Conversely, the park's nonmigratory species, such as the Carolina chickadee, tufted titmouse, and downy woodpecker, have either increased or remained steady.

The situation in Rock Creek Park would be less troubling if it were an isolated case. But, in fact, most of the parks with long-term census data show similar declines in migratory songbirds. The forests of the Greenbrook Sanctuary in New Jersey, for example, harbored dozens of ovenbirds and redstarts in the 1950s; both species are now gone, and populations of other migrants in the sanctuary have dropped precipitously. Cabin John Island, in the Potomac River just outside Washington, D.C., has lost roughly half its migratory songbirds since the late 1940s.

These long-term census data indisputably show a sharp decline in populations of migratory songbirds, but we are talking about a handful of places, most of which are isolated parks surrounded by cities and suburbs. Does the situation in Rock Creek Park or Greenbrook Sanctuary reflect a broader pattern of decline among migratory songbirds, or is it purely a local issue?

Ornithologist Sidney Gauthreaux would argue that something bigger is afoot. For decades he has monitored bird migration using radar. Every spring and fall, millions of migrating songbirds cross the Gulf of Mexico, en route to their breeding or wintering grounds. So many birds pass over the gulf on certain nights that they appear as fast-moving blotches on weather radar screens. Although it is impossible to count the actual number of birds from these radar images, it is possible to tally the number of blotches and derive a crude measure of the extent of the migration. And because all the radar screens are regularly photographed and the images archived, one can look for trends. Comparing images from radar stations in Louisiana and Texas for the years 1963–67 and 1987–89, Gauthreaux came to a startling conclusion: during this period, the number of flights of migratory birds had dropped by nearly 50 percent. Moreover, in the 1960s, large numbers of songbirds were migrating almost every night when weather conditions were favorable. By the 1980s, such large flights were occurring on only 36–53 percent of the acceptable flying nights.

Gauthreaux's study would appear to provide solid evidence of a long-term decline in migratory songbird populations—what better evidence could there be than a measurement of the actual migration? —but even so, there are confounding factors that make simple interpretations risky. For example, the data analyzed thus far come from only two weather stations along the gulf. Is it possible that the other stations were recording higher than normal numbers of migrants? Might the birds have shifted their migratory pathways, and, if so, could this account for the observed decline? Were the late 1980s an unusually bad time for migratory birds? No one can be certain.

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Excerpted from No Way Home by David S. Wilcove, Louise Zemaitis. Copyright © 2008 David S. Wilcove. Excerpted by permission of ISLAND PRESS.
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