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The vast, unbroken beach at Bahía Lomas stretches for about 30 miles along the Strait of Magellan in Tierra del Fuego, at the southern end of South America. I’ve stood for hours hoping to see shorebirds in this remote, inhospitable place, where the wind blows at hurricane strength, the trees are forced into a permanent lean, and the ebbing tide disappears beyond the horizon, about four miles away. When it returns, rushing in over the wide mudflat, shorebirds follow, thousands of them, appearing first as puffs of smoke in the distance, then in large flocks, rising and falling in smooth, sinuous curves. They alight on the mud, and I am surrounded by birds.

They are mostly Red Knots, sandpipers that have come to winter on this vast beach. Slightly larger than robins, Red Knots travel some 19,000 miles every year, sometimes flying for six or eight days at a stretch without stopping to rest or feed. Their marathon journey, from one end of the Earth to the other and back again, distinguishes the Red Knot as one of the avian kingdom’s most accomplished fliers. Ornithologists have long recognized the knot as sublime. Alexander Sprunt Jr., preeminent South Carolina ornithologist, writer, ardent conservationist, and, from 1935 to 1973, supervisor of the ̽����ѡ’s southern sanctuaries, saw more than his share of charismatic birds, including colorful Carolina Parakeets and fabled Ivory-billed Woodpeckers. To Sprunt and his colleague E. Burnham Chamberlain, writing in the 1949 edition of South Carolina Bird Life, the small, less conspicuous knot nonetheless held a special place, representing “an untrammeled wildness and freedom that is equaled by few and surpassed by none.” 

Curious about how such small birds can manage such extraordinary journeys, I followed them—from the treacherous shoals along the Magellan Strait, to a crowded resort in Argentina, up along the East Coast of the United States, into the icy Arctic, and then back, along the muddy shores of James Bay, through the dense fog of Quebec’s Mingan Islands, and then into the bay behind my home in Massachusetts. I found them there one autumn day—young birds that a couple of months earlier had walked half a mile, perhaps a mile, from their nests to the sea, and then, inconceivably, begun their first long migration, along a route they’d never traveled, to a destination they’d never seen. The story—of their tenacity and the tenacity of the hundreds of people I met along the way dedicated to providing the birds safe passage—became , The Narrow Edge: A Tiny Bird, an Ancient Crab, and an Epic Journey.

Just as the book was going to press, the U.S. Fish and Wildlife Service listed the rufa Red Knot as threatened under the Endangered Species Act, the first U.S. bird listed explicitly because its existence is imperiled by global warming. The knot they describe, Calidris canutus rufa, is one of six subspecies of Red Knots worldwide, distinguished by its Eastern Seaboard migration. Since 2000 the rufa Red Knot’s population has declined by roughly 75 percent at key stopovers. Threats to the bird, according to the USFWS, are likely to put the rufa Red Knot “in danger of extinction in the next few decades.” 

One might think a bird that each year flies the length of the globe and back—a bird that finds sustenance and shelter in places as widespread and diverse as the Jersey Shore, the Arctic Circle, and the Magellan Strait—might be immune to the warming planet; might, if one of its homes or stopovers becomes unsuitable, simply find another. It’s more complicated than that. 

In 2014 the ̽����ѡ’s science team published the results of a seven-year study showing how global warming might affect North America’s bird species in the coming decades. The showed that of 588 North American bird species considered, 314 of them could lose more than half their range by 2080. That means that for these birds, the area with the climate conditions they need to survive will shrink or shift so dramatically that they’ll be left with less (often far less) than half of the suitable range that exists today. It’s an alarming prediction, particularly when there’s no guarantee the birds will find new habitat elsewhere.

Shorebirds spend their lives at the delicate places where sea meets land—one of the front lines of climate change—and are therefore particularly vulnerable. In 2014 scientists led by Michael Reed from Tufts University and Hector Galbraith, then at the Manomet Center for Conservation Sciences, reported that global warming for nearly 90 percent of North American shorebirds. The team evaluated threats the birds may face in a warming world, including a shrinking tundra; rising seas; ocean acidification; increasingly stormy weather; and dependence on specialized environments, such as Delaware Bay. In their analysis, even at its lowest sensitivity, 20 populations of North American shorebirds, including the Red Knot, would fall into the U.S. Shorebird Conservation Plan’s highest-risk category—“highly imperiled.” As their world heats up, Red Knots are threatened almost everywhere along their flyway: The warming, acidic sea inhibits the growth of the shellfish the birds need to fuel their impressive migration; rising seas may flood their seaside homes; rising temperatures threaten to shrink their Arctic nesting grounds and expose them to more predators. No matter where they go, no matter how many new homes they might seek, Red Knots can’t escape the effects of global warming. 

To power their long migrations, these tiny birds require phenomenal quantities of energy-rich food. Along the route, they gorge on tiny mussels and clams, horseshoe crab eggs, and sea worms, packing in energy for the flights ahead, burning it off on the wing, and then refueling again at the next stop. In Delaware Bay they nearly double their weight—a metabolic feat that would likely make any human attempting it seriously ill but that crowns knots as powerhouse long-distance fliers and one of the animal kingdom’s most rapid and efficient energy consumers. Ocean acidification, a largely invisible consequence of global warming, may soon compromise the quality of Red Knots’ food, if it hasn’t already.

As carbon dioxide emissions rise, the pH of seawater drops, and it becomes less saturated with aragonite, a mineral clams, mussels, oysters, and scallops require for building strong shells. In increasingly acidic seawater, these animals’ shells are smaller, thinner, and weaker, and their larvae grow more slowly, with fewer surviving to settle on the sea bottom. Mussels are unable to cling to their homes on rocky tidal flats. In a 2012 symposium, the International Geosphere-Biosphere Programme, UNESCO’s Intergovernmental Oceanographic Commission, and the Scientific Committee on Oceanic Research concluded with “high confidence” that mollusks are “one of the groups most sensitive to ocean acidification.” Ocean acidity is increasing rapidly—more than 10 times faster than at any time in the past 55 million years, and possibly at a rate unprecedented in the past 300 million years. Red Knots weren’t around then; they don’t benefit from an evolutionary history of adapting to such rapid changes in ocean chemistry and the problems it brings to the shellfish that, in most places along the flyway, are their primary source of food. 

Near the city of Río Grande, Tierra del Fuego, wintering knots once feasted on a cornucopia of soft-shell clams and blue mussels, consuming energy at rates higher than at almost any stopover along their migration except Delaware Bay. By 2008 Río Grande had grown significantly, bringing more people to the beach, disturbing the birds as they fed. In addition, their prey had become smaller. In the hours when the birds fed, before tides covered the flats, their intake of this once exceptional food source plummeted to one of the lowest along the route. At Bahía Lomas, concerned researchers discovered that clams—the knots’ main prey here—had become inexplicably small: barely a quarter of an inch, and one-fourth the size of the same species the birds eat on some tidal flats in Argentina. Funding is critically needed to determine the role ocean acidification plays in this alarming development.

Farther north, in James Bay, Ontario—where more than 10 percent of Red Knots stop to refuel on the journey south—an increasingly acidic sea doesn’t bode well either. I tracked birds here from a camp, reachable only by train and then helicopter, on the western shore of this isolated, remote bay, where knots flew in to fill up on a tiny clam, Macoma balthica. In the late 1970s Canadian scientists analyzing the clam’s distribution found it packed into the tidal flats here at densities as high as 9,000 per square meter. They also found that as pH dropped, clam density decreased. In 2014 another group of scientists found surface waters in James Bay undersaturated in aragonite and concluded that investigations into the biological implications were “urgently needed.” Across the Atlantic, in Northern Europe, scientists are finding that as the pH of the Baltic Sea drops, the number of larval Macoma balthica growing large enough to survive and settle on the bottom is projected to decline by at least a third.

Ocean acidification isn’t limited to high latitudes. Where rivers meet the sea, fresh water and polluted runoff can amplify ocean acidification. A team of 17 scientists led by the University of California at Davis along the Eastern Seaboard vulnerable to acidification—in coastal Massachusetts, Long Island Sound, Virginia, and South Carolina—many overlapping with critical feeding grounds for knots.

It’s not just acidification that takes a toll on the knots’ food; warming water can as well. Each spring on the barrier islands of Virginia’s Eastern Shore, some 13,000 knots stop to feast on young mussels that have just settled on the islands’ peat banks. Here mussels are already at the southern end of their range, which, as ocean temperatures rise, is shifting north—by more than 200 miles over 50 years. In the years ahead, the mussels’ drifting larvae may no longer reach these islands. The Red Knot migration to the Arctic is like a ladder, with every rung now a critical way station. Loss of only a few more footholds, like the mussels in Virginia, could compromise the entire journey. Bryan Watts, director of Virginia’s Center for Conservation Biology, puts it this way: “Reducing the menu for a species is rarely a good thing. The loss of blue mussels that appear to be a critical energy source for knots staging in Virginia will make the task of putting on enough fat to reach the Arctic in breeding condition much more challenging.”

Global warming reverberates along the flyway, as rising seas threaten to swallow the marshes and beaches where knots forage and roost. Though the sea has been rising along the eastern United States ever since the ice ages came to a close, today’s accelerated pace, coupled with human development, stops the shore from naturally moving inland. Houses, roads, and other seaside development—along with the bulkheads, seawalls, and jetties designed to protect them—can increase erosion, impeding the inland migration of beaches.

Jetties, dams, and river diversions north of South Carolina’s Cape Romain National Wildlife Refuge have starved its low-lying beaches of sand, and over decades the losses have come to exceed the naturally occurring gains. Bird Island, where knots, skimmers, and other birds once roosted, now floods on incoming tides. Another island, packed with skimmers, terns, and pelicans, where 3,000 knots have roosted, is also disappearing; water laps at a sign where a dune once stood.

Not that there aren’t places where crucial habitat has been maintained. In Georgia, the Altamaha River flows unimpeded to the sea, carrying sand that rebuilds shoals and bars as they shift with the rising water. Much of the delta is protected from development by conservation easements, leaving the beaches room to retreat and reform. Walking one of those beaches early one morning—a beach that had widened 200 feet in five years—I came across a flock of knots. Their uncanny ability to locate the richest, most plentiful food brought them to this place where only the night before horseshoe crabs had come ashore to spawn.

The threat of rising water comes into stark relief in Delaware Bay, the East Coast’s Serengeti of spring shorebird migration. Here Hurricane Sandy destroyed about 70 percent of New Jersey’s horseshoe crab spawning beaches. While scientists can’t say whether Sandy was a direct consequence of global warming, they did find that in 1950 the likely frequency of a Sandy-scale storm surge on beaches around Atlantic City and south, including Delaware Bay, was perhaps once every 100 years; by 2012 those odds had increased to once every couple of decades. Just after the storm, wildlife organizations removed 800 tons of debris and added 45,000 tons of sand along Delaware Bay, repairing horseshoe crab spawning beaches, buying birds and horseshoe crabs time as the water rises.

At the end of the knots’ long migration lies their nesting ground, thousands of square miles spread throughout central Canada’s remote Arctic tundra, where summer is short. An abundance of insects, fewer parasites, and fewer predators offset the tremendous costs of the strenuous migration here. Safety is particularly important—over the distance represented by 29 degrees of latitude, nest plundering drops 65 percent. A warming Arctic jeopardizes that protection.

I accompanied the shorebird team from Canada’s National Wildlife Research Centre to Southampton Island, just south of the Arctic Circle. There Red Knots nest on barren gravel ridges, affording them a clear view of their most dangerous predators: jaegers and Arctic foxes. Both these predators also eat lemmings—furry rodents whose populations rise and fall dramatically in three-to-five-year cycles. If lemmings had been plentiful, we would have found them in our boots, heard them scurrying through the cabin at night, and seen their burrows in the grass. We didn’t. When lemming numbers are down, jaegers and foxes prey more frequently on bird eggs. We often saw Arctic foxes trotting along the ridges, bringing stolen eggs to their caches. Where lemming cycles have been studied in other parts of the Arctic, the Intergovernmental Panel on Climate Change attributes their collapse, with “high confidence,” to global warming. The U.S. Fish and Wildlife Service found that while Red Knots have evolved to weather the cyclical rise and fall of lemmings, dampening the “boom” part of the cycle poses a “substantial threat,” jeopardizing the future of new generations of Red Knots and the long-term survival of the population.

As the world warms, Red Knots may lose their Arctic homes. On the open, stony ridges where the birds now nest, vegetation is sparse—the occasional cluster of low-lying purple saxifrage brightens the gray landscape, and a few inches-high willows are scattered amid the rock. But as the Arctic turns greener, the tree line will move north and the tundra will recede. Barren ridges where I walked every day tracking shorebird nests will give way to shrubs and then woods. As lichens and moss yield to alders, taller willows, and birches, the tundra may shrink to its smallest size in 21,000 years. Squeezed up against the Arctic Ocean, much of the knots’ tundra breeding ground may disappear. The birds, already living at the northern end of Earth’s northern lands, don’t have much room to retreat.

In Delaware Bay, I held a knot, feeling its heart beating against my fingers, cradling in my palm a small bird that had come so many miles, with so many more to go. Kayaking out onto the sandbars in the bay behind my home on spring and fall’s rising tides, I’ve waited for hours watching for shorebirds pausing to rest on their long, extraordinary journeys. As they lift off, I wish them fair winds, hoping that they will live to return to these waters another year, and that their numbers will rebound. Many people, season by season and beach by beach, are working to restore the Red Knots’ seaside homes, to ensure that this icon of “untrammeled wildness and freedom” will once again have safe harbor and food in abundance. We share a home with Red Knots along the sea edge. As the consequences of global warming begin to resonate along the flyway, how we face what is perhaps the greatest ethical crisis of our generation will determine not only their fate but the fate of millions of other shorebirds.