Book ‘A World on the Wing’ by Scott Weidensaul

PDF Excerpt 'A World on the Wing' Book by Scott Weidensaul
The Global Odyssey of Migratory Birds
An exhilarating exploration of the science and wonder of global bird migration. In the past two decades, our understanding of the navigational and physiological feats that enable birds to cross immense oceans, fly above the highest mountains, or remain in unbroken flight for months at a stretch has exploded. What we’ve learned of these key migrations―how billions of birds circumnavigate the globe, flying tens of thousands of miles between hemispheres on an annual basis―is nothing short of extraordinary. Bird migration entails almost unfathomable endurance, like a sparrow-sized sandpiper that will fly nonstop from Canada to Venezuela—the equivalent of running 126 consecutive marathons without food...
Publisher: Farrar, Straus and Giroux (March 16, 2021)  Hardcover: 400 pages  ISBN-10: 0374159270  ISBN-13: 978-0374159276  ASIN: B08BVTBHF9

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Scott Weidensaul is the author of nearly thirty books, including Living on the Wind, a finalist for the Pulitzer Prize. A writer and researcher specializing in birds and bird migration, he is a native of Pennsylvania now living in New Hampshire.

Book excerpt


Tundra may be the most gloriously comfortable mattress in the world. A little damp, it’s true, which is why it’s a good idea to wear rain pants and a jacket, even on a clear, chilly morning like this one—the sun just touching the peaks of the Alaska Range with pink-orange light, the glacier-wrapped bulk of Denali a vast, rosy monolith 70 miles to our west, uncharacteristically free of clouds.

My three companions and I flopped down with happy sighs, legs outstretched and hands laced behind our heads, onto the soft, spongy cushion of sphagnum moss, dwarf cranberries, reindeer lichen, and other Lilliputian tundra plants. The break felt good. We’d risen at two in the morning, in the bright twilight that passes for the middle of the subarctic night in the interior of Alaska. By three, keeping an eye out for moose or grizzlies, we were headed west along the 90-mile gravel road that bisects the six-million-acre wilderness of Denali National Park and Preserve. We never knew what we’d see. The day before, a large male wolf had trotted warily around our National Park Service truck before sniffing nervously at the rear fender, just a few feet from my open window.

There were no such interruptions today. By four o’clock, 30 miles inside the park, we’d shouldered our packs and bundles of aluminum net poles, then trudged down a long slope to a sinuous willow thicket that snaked through a mile-long draw. As luxurious as spongy tundra is to lie on, it is a tiring chore to hike across, with every footstep sinking deep or rolling on some hidden tussock, while shin-high birches and willows claw at your feet and legs.

“Hey! Hey!” we yelled, to alert any moose or grizzly bear that might be hidden in the dense, 10-foot-high brush ahead. “Blah blah blah blah!” I shouted nonsensically; it doesn’t matter what you bellow, just so you don’t surprise a protective cow moose with a calf, or startle a grizzly whose first reaction might be to charge. Unlike many hikers, one thing we never did was yell, “Hey bear!” Those words, old Alaskan hands will tell you, should be reserved solely for the gut-twisting moment when a grizzly pops up at close range—a warning to the bear, but more importantly to everyone else in earshot.

As it was, all we alerted was a family of willow ptarmigan, half a dozen rotund, brown fledglings that boomed off in as many directions while the mother grouse barked her displeasure. We shucked our loads, and I followed Laura Phillips, the park’s avian ecologist, as she wormed her way into the seemingly impenetrable tangle of willows. Somehow, the moose had no trouble maneuvering in there—the wet ground was pocked with their saucer-sized tracks and piles of oblong droppings. But in the middle we found a slender lozenge-shaped meadow just a few yards wide, blue with the stately flowers of monkshood and larkspur, its margins purple with spires of fireweed.

We weren’t looking for ptarmigan or wildflowers, though, but for thrushes—and not to watch, but to catch. After more than three decades of visiting Denali, I was helping to launch a new research project there to better understand the lives of the park’s birds, which every year fan out across three-quarters of the earth’s surface on their migrations.

Soon, we had three 40-foot-long mist nets radiating out into the brush. David Tomeo, with Alaska Geographic, and seabird biologist Iain Stenhouse—a transplanted Scot now living in Maine, who was once Audubon’s director of bird conservation in Alaska—secured the net poles with guy lines of bright red parachute cord. I jammed a long wooden dowel into the ground mid-net, and perched on its tip a painted, life-sized wooden thrush decoy. Then I thumbed the controls of a battered old MP3 player, from which emerged the buzzy, ethereal song of a gray-cheeked thrush. Our work finished for the moment, the four of us walked 10 or 15 yards up the hill, out of the willows and into the open tundra, and sank down to relax for a few minutes. Our hope was that a male thrush—hearing what sounded like an intruder in his jealously defended territory—would come barreling down through the shrubs and collide harmlessly with our delicate nets. Then we could carefully attach a tiny device called a geolocator, weighing barely half a gram, to the small of his back. For the next year, it would record the bird’s location as it flew to South America and back, giving us the first glimpse anyone’s had into the specifics of this bird’s epic migration.

For the better part of a century, the only means scientists had of figuring out where birds traveled was by putting lightweight numbered bands on their legs, and hoping to hear if the banded bird was ever encountered again. Banding is still a critical element of migration research—some 7 million mallard ducks have been banded in the past century, for example, and 1.2 million of them recovered (mostly by hunters), providing data that help underpin our very successful management of waterfowl populations. But it’s a long, slow slog if you’re studying a rarely banded bird in a remote area—a bird that, unlike mallards, isn’t legally hunted. In the past century, roughly 82,000 gray-cheeked thrushes have been banded in North America as a whole, but only 4,312 of them were in Alaska—and of those banded Alaskan thrushes, only three have ever been encountered again. One was caught close to where it was banded, one on its spring migration north through Illinois, and one heading south in the fall in Georgia. That’s not much to go on.

What banding data and observations we do have show that gray-cheeked thrushes are exceptionally long-distance migrants. Even though they weigh only about 30 grams—a shade more than an ounce—they travel from conifer forests and thickets in northern Alaska and the Canadian subarctic to South America and back each year. At least some of them cross the Gulf of Mexico in a 600-mile nonstop leap, while others may follow the long finger of Florida and then overfly the Caribbean. In winter, they disappear into the rain forests of northern South America, but we have only the sketchiest notion of where they go within that vast continent.

But where banding struggles to fill in the blanks, newly miniaturized technology is opening exciting horizons in the study of bird migration. The geolocators we were using are just one example of tiny, relatively inexpensive tracking devices that are revolutionizing migration research. Instead of depending on satellite transmitters that cost $4,000–$5,000 each (and which are, in any case, far too heavy for small songbirds), our geolocators weigh a fraction of a gram and cost just a few hundred dollars each. Our team, headed by National Park Service ecologist Carol McIntyre, was starting a multiyear project to trace the migratory links between Denali and the far corners of the globe to which the park’s birds fly. Our geolocators would give us the first opportunity anyone’s ever had to track the actual route and destinations of the park’s thrushes.

But first we had to catch some. We’d had easy success the previous week tagging Swainson’s thrushes, which are abundant in the spruce forests of Denali. The closely related gray-cheeked thrushes, on the other hand, were proving to be a little more challenging, and we hoped that the use of a few extra nets might make a difference that morning.

The tundra was almost too comfortable, and after about 15 minutes of waiting and dozing, I levered myself off the ground and trotted down the hill to the willows to see what we had caught. In one net, a male blackpoll warbler hung head-down in the cushioning mesh—another bird that makes an extraordinary migration, from Alaska to the Atlantic coast of Canada and the northeastern United States, then south in a nonstop 90-hour flight over the western Atlantic to South America. The next net held a male Wilson’s warbler, tinier even than the blackpoll, weighing just nine grams, less than a third of an ounce. Those Wilson’s warblers that breed in central Alaska migrate (we think) to the Gulf Coast of Texas and eastern Mexico and south into Central America. Many of them may commute to the Yucatán Peninsula across the Gulf of Mexico—but no one really knows. Only a single Wilson’s banded in the Alaskan interior has ever been recovered away from the breeding grounds, and that one was found in Idaho on its way south.

We would be tagging blackpolls and Wilson’s warblers another time, but for the moment, that mystery would have to wait, and I released them quickly. Our focus this morning was thrushes, and to my disappointment, there were none in our nets. I turned to trudge back up the hill—and in that moment, the quiet of the morning became a terrifying chaos.

“Hey bear! HEY BEAR!” Laura and David’s voices had the rough edge of panic, their arms raised and waving wildly against the pale dawn sky. I couldn’t see Iain, hidden from me by the willows.

I heard a huffing, staccato roar, and an explosive wooden sound like someone pounding two-by-fours together, which I realized were the clashing jaws of an angry grizzly “popping” its teeth in rage. Time, as often happens in moments of extremity, seemed to slow. I couldn’t see the charging bear, but assumed it was coming out of the willows where I was standing. I froze.

“HEY BEAR!” The roaring and popping sounds were much closer now, and the thicket was filled with the crashing of a large animal, very close and moving very fast. David bellowed, “Scott, get the hell out of there!”

I bolted from the willows as the bear passed a few yards away, so close that I could hear its ragged, woofing breaths and smell its pungent odor, but invisible behind the screen of brush. In seconds I scrambled back up the hill to my friends. Turning, we saw the bear—a big female with a dark yearling cub in tow—burst out the far side of the willows and race away from us with the horse-like speed for which grizzlies are famous. The sow’s straw-blonde fur rippled as she pounded up the far tundra slope, vanishing over the crest.

The story emerged in shaky, disjointed pieces. Everyone was still lying down when the bear emerged from a hidden draw, just 50 or 60 feet away and a little behind them. “I looked over to say something to Iain,” Laura said, “and I saw this grizzly head beyond him. I said, ‘Oh shit.’ We started to stand up, and she just charged.”

Iain was closest. “I heard you and David yell, but I couldn’t move,” he said in his Glasgow accent, shaking his head. “I was just—I couldn’t move.” The grizzly crossed the distance in seconds. Only a few feet from Iain, the bear changed its mind; Laura and Iain both said they could see the fraction of a moment when the sow decided not to maul them—and turned instead to race down the hill, directly toward me.

“It’s ironic,” David said, “that the one person who didn’t see the bear coming is the one who was probably in the greatest danger of getting mauled.” It took me a second to realize he meant me. Even for an angry grizzly, three people together is a lot to tackle. But alone and hemmed in by the willows, I would have been helpless if she’d spotted me just a few yards from her in the thicket, and had decided to vent her frustration and fear.

Laura drew a long, uneven breath and looked around. “You guys think we have any nets left?”

The bears’ path had been right through the middle of our array, but somehow the 400-pound sow and her cub had missed them. And whether because of all the commotion, or because they’d fallen to the lure of the recorded song in spite of it, there were three gray-cheeked thrushes hanging in the mesh. Knowing the bears were safely gone —and with a sense of relief that we had something else to think about—we set to work.

Placing the birds in lightweight cloth holding bags, we spread a small tarp on the damp ground and laid out our tools—banding pliers, clipboard, a spring scale, a small camera, and the first geolocator. The device was maybe a third of an inch long, with a short plastic stalk poking out its rear that carried a light sensor. Small elastic loops stuck out to either side like rabbit ears. Laura removed the first thrush, gently caging it in her hand with its neck between her first two fingers. Gray-cheekeds are two-thirds the size of robins, lovely in their subtlety. Their upperparts are a cool olive-gray, their off-white chests covered in brownish spots that look like watercolor gently seeping into thick paper. Attaching the geolocator took less than a minute. Iain worked an elastic loop high up one leg, to the top of the bird’s thigh. With her thumb, Laura steadied the geolocator in the small of the thrush’s back while Iain slid the other loop up the opposite leg; thus secured, the tracker rode snugly just above the bird’s rump, all but the light stalk hidden beneath its back feathers.

With practiced moves, Laura banded the thrush—a standard metal band on the right leg, and two colored plastic bands, yellow above orange, on the left. When Denali’s migrants returned the next spring, the color bands would make it easier to relocate this and the other tagged thrushes so we could recapture them, remove the geolocators, and download their data. One by one, we processed and released the thrushes, each of which flew back into the sheltering willows with nasal, scolding jee-eer notes. We packed up the gear, but as we rose to go, I realized Iain was staring out over the hills, in the direction where the bears had gone.

“You know what?” he said, his bright smile conveying a sense of cheerful discovery. “I didn’t think my sphincter muscle was that strong!”

For almost six years in the 1990s, I followed birds up and down the Western Hemisphere, exploring the phenomenon of migration for a book called Living on the Wind. I’d come to the subject largely as a deeply interested observer—a lifelong birder who had, a decade or so earlier, become obsessed with banding raptors. Initially, I must admit, the attraction to banding was largely the adrenaline-surging thrill of luring a goshawk or golden eagle out of the sky and into my nets—fly-fishing in the air, on an epic scale, for prey with talons and a regal mastery of the wind. But with each hawk or falcon on whose leg I placed a band—and with each time one of those marked birds was recaptured or found dead in some distant place, adding a little more to our understanding of their migrations—I became more fascinated with the natural forces that push not just powerful birds of prey but even the tiniest and seemingly most fragile warbler to cross immensities of space with a speed and physical tenacity that beggars human imagination.

In the past two decades, science’s understanding of migration—of the mechanics that allow a bird, alone and on its first journey, to find its way across the globe in the face of crosswinds, storms, and exhaustion—has exploded. To take just one especially mind-bending example, we’ve known since the 1950s that birds use the earth’s magnetic field to orient themselves. Ornithologists long assumed this ability was a sort of biological compass, and the presence of magnetic iron crystals in the heads of many birds seemed to bear this out—except that those magnetite deposits actually appear to play little role in orientation. Vision, quite unexpectedly, does. Expose a bird to red wavelengths instead of natural white light, and it loses its ability to orient magnetically, regardless of what minute lumps of iron might be in its head. But just why this should be has baffled ornithologists since at least the 1970s.

It now appears that birds may visualize the earth’s magnetic field through a form of quantum entanglement, which is just as bizarre as it sounds. Quantum mechanics dictates that two particles, created at the same instant, are linked at the most profound level—that they are, in essence, one thing, and remain “entangled” with each other so that regardless of distance, what affects one instantly affects the other. No wonder the technical term in physics for this effect is “spooky action.” Even Einstein was unsettled by the implications.

Theoretically, entanglement occurs even across millions of light-years of space, but what happens within the much smaller scale of a bird’s eye may produce that mysterious ability to use the planetary magnetic field. Scientists now believe that wavelengths of blue light strike a migratory bird’s eye, exciting the entangled electrons in a chemical called cryptochrome. The energy from an incoming photon splits an entangled pair of electrons, knocking one into an adjacent cryptochrome molecule—yet the two particles remain entangled. However minute, the distance between them means the electrons react to the planet’s magnetic field in subtly different ways, creating slightly different chemical reactions in the molecules. Microsecond by microsecond, this palette of varying chemical signals, spread across countless entangled pairs of electrons, apparently builds a map in the bird’s eye of the geomagnetic fields through which it is traveling.

That’s by no means the only gee-whiz discovery. Researchers have found that in advance of their flights, migrant birds can bulk up with new muscle mass without really exercising, something humans would love to copy. Because a bird’s muscle tissue is all but identical to a human’s, the trigger must be biochemical, but remains a tantalizing mystery. They also put on so much fat (in many cases more than doubling their weight in a few weeks) that they are, by any measure, grossly obese, and their blood chemistry at such times resembles that of diabetics and coronary patients—except that they suffer no harm. Nor do birds flying nonstop for days suffer from the effects of sleep deprivation; they can shut down one hemisphere of the brain (along with that side’s eye) for a second or two at a time, switching back and forth as they fly through the night; during the day, they take thousands of little micronaps lasting just a few seconds. Researchers have found dozens of similarly extraordinary ways in which a bird’s body copes with and overcomes the stress of long-distance travel.

And as science’s grasp of the mechanics of migration has improved, so too has our understanding of the gritty, life-and-death challenges that increasingly face these travelers, and the almost inconceivable feats they accomplish twice each year to reach their destinations. In the past two decades we’ve realized how badly we have underestimated the simple physical abilities of birds.

Until recently, the acknowledged long-distance migration champion was the Arctic tern, a ghostly gray seabird the size of a dove, which breeds in the highest latitudes of the Northern Hemisphere and winters in the southern oceans between Africa, South America, and Antarctica. Draw lines on a map between those waypoints, scratch a few calculations on a table napkin, and you reach the conclusion that generations of ornithologists had—that Arctic terns migrate some 22,000 to 25,000 miles each year. It was a guess, because tracking technology wasn’t nearly small enough for a delicate creature like the tern to carry. But as transmitters and data-loggers began to grow smaller, they could be deployed on other, somewhat bigger seabirds—which soon left the Arctic tern’s assumed record in the dust.

In 2006, scientists using geolocators announced they had successfully tracked 19 sooty shearwaters from their breeding colonies in New Zealand. Even a “local” feeding run during the breeding season, when the parents forage for squid and fish to bring back to their nest burrows for their chicks, carried these plump, dark gray birds from New Zealand down into the frigid sub-Antarctic waters thousands of miles away, and back. Once the chicks fledged, however, they and the adults all headed north, crossing the equator to reach “winter” feeding grounds in the boreal summer off Japan, Alaska, or California. By following wind and ocean currents in looping curlicues across the Pacific, the birds (in the words of the researchers) enjoyed “an endless summer.” It’s a helluva road trip, since the routes taken by some shearwaters exceed 46,000 miles a year.

Finally, by 2007, geolocators had grown small enough that my Scottish friend Iain and several of his colleagues were able to attach them to the legs of Arctic terns in Greenland and Iceland. A year later, the returning birds were recaptured, and the story that unspooled from their stored data was astonishing.

The first surprise was that the terns took one of two dramatically different tracks south, regardless of their colony of origin. Some veered east to the northwestern bulge of Africa, then angled back across the narrowest part of the Atlantic to the coast of Brazil before continuing south to the Weddell Sea along the Antarctic Peninsula. In spring they migrated to the waters off southern Africa, then across the Atlantic again to northern South America, and finally on to the North Atlantic—a figure eight, inscribed on the planet by endlessly beating wings. For some reason, other terns from the same colonies instead shadowed the coast of Africa almost to the Cape of Good Hope, then either crossed the Southern Ocean to the Antarctic coast, or followed the screaming gales of those high, storm-raked latitudes for thousands of miles farther east, south of the Indian Ocean.

In all, Iain and his colleagues found that even the least ambitious of their terns migrated at least 37,000 miles a year, though some traveled almost 51,000 miles a year—a new long-distance record, and more than twice what scientists had once assumed was possible for this species. And just to cap that, three years later researchers who had tagged Arctic terns in the Netherlands found that those birds were traveling up to 57,000 miles a year, reaching the waters off Australia and using staging areas in the Indian Ocean (where, it turns out, tagged terns from the coast of Maine also gather). Any seabird biologist will admit, especially after a beer or two, that no one really has a clue what the true limits of tern migration might be.

Many other assumptions about migration have been turned on their heads in recent years. It’s the nature of the beast; ecology is an almost perversely complicated subject, and every layer of the onion that we peel back just reveals further complexities.

Twenty years ago, North American ornithologists who had assumed the biggest challenge for migratory songbirds lay in the loss of wintering habitat from tropical deforestation were coming to grips with a problem much closer to home. A growing body of research showed that forest fragmentation—the endless slicing of large, intact tracts of woodland into smaller and smaller scrubby shards, bisected by roads, utility corridors, developments, and fields—posed a serious danger to many of the most prized and lovely migrant songbirds, like tanagers and thrushes, which evolved to nest in unbroken woodland. Fragmentation, it turns out, brings a host of evils. They include so-called edge predators that thrive in disturbed habitats, creatures like raccoons, skunks, opossums, grackles, crows, jays, and rat snakes—all adept nest predators that are rare or absent from deep woods. Fragments also invite brown-headed cowbirds, grassland birds that parasitize the nests of other songbirds (and which were originally restricted to the Great Plains). What’s more, fragmentation dries out the very forest itself, reducing insect abundance and creating other environmental challenges for the nesting birds.

Scientists have tracked the nesting success of so-called forest-interior songbirds like wood thrushes, monitoring their nests to see which ones produce the most eggs, and how many eggs successfully grow into fledglings that fly off on their own to form the next generation. Decades of such study confirm that when big expanses of woodland are chopped into smaller fragments, nesting success drops in lockstep with the splintering of the forest.

So, to save the bird, save the forest. While preventing fragmentation is challenging in practice, it’s a simple target to articulate and to aim for, and one that has guided important elements of bird conservation since the 1980s. But—and in ecology, there is usually a but lurking in the underbrush—more recent research has uncovered a real surprise. This one came when scientists took the next step. Instead of just monitoring breeding success in those safe, intact forests, they began the much more arduous task of tracking fledgling thrushes after they left the nest and scattered to the four winds. When they put tiny radio transmitters on those adolescent thrushes and followed them until they were ready to migrate, the researchers found that many of the juveniles abandon the mature, expansive woods where their parents nested—the intact forests we’ve come to assume were of singular importance to their survival, and whose preservation has been a major focus of migrant conservation.

For a month or more leading up to migration, when the young songbirds must rapidly gain weight so they can make the exhausting flights to Latin America or the Caribbean that lie ahead, the fledglings congregate instead in scrubby, brushy, early successional thickets—the kind of habitat created, say, after a clear-cut has begun to regenerate, a clear-cut that might otherwise be seen as destroying habitat for forest-interior birds.

It’s not that these birds don’t need contiguous forests—they do. But that’s not all they need. Time and again, science has underestimated the complexity of migratory ecology.

This isn’t willful ignorance; studying small, active creatures whose annual migrations cover tens of thousands of miles is inherently, extraordinarily difficult. But in a lot of ways that are not uncommon in science, ornithology has always been a victim of blinkered vision and the path of least resistance. For the better part of two centuries, ornithologists were almost exclusively North American or European—and because it’s easiest to study something close to where you live and work, for a long while we therefore knew mostly about the lives of migratory birds during the few months when they were on their temperate breeding grounds. In the 1970s and ’80s that began to change, and the emerging research from the tropical wintering grounds upended many comfortable assumptions about migrant ecology. Once thought to be adaptable, go-along-to-get-along types that could fit into any vacant slot in the tropics, many migrants proved to be every bit as specialized as the resident birds with which they shared the landscape, tightly bound to specific, often narrow ecological niches. Even within the same species, scientists found, different age and sex classes often had dramatically different needs, and used very different regions or habitats—adult males preferring dense rain forest, for example, and juvenile females drier, scrubbier habitat.

This new understanding came as alarm bells were sounding over rampant tropical deforestation, which quickly came to be seen as the greatest threat to neotropical songbirds. Conversely, neotropical migrants like warblers and tanagers simultaneously became poster children for the campaigns of the 1980s and ’90s to save the rain forest, the most direct (and emotionally resonant) link between a distant and threatened ecosystem and American backyards.