Today we’re sharing a story from Audubon Magazine written by Deborah Cramer, author of The Narrow Edge: A tiny bird, an ancient crab, and an epic journey. The tiny, threatened red knot is an omen for how devastating ocean acidification can be.
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 National Audubon Society’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.”
Red knots at Mispillion Harbor, Delaware. Delaware Bay hosts the largest concentration of the rufa red knot subspecies during the spring, when knots on their marathon migrations stop to refuel and take advantage of the largest gathering of horseshoe crabs in the world. Credit: Gregory Breese/USFWS
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 my 2015 book, 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 National Audubon Society’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 Audubon Birds and Climate Change Report 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.
A red knot parent and chicks near Hudson Bay (northeastern Canada). The females generally depart a few days after the eggs hatch, leaving the males to tend the young until they’re old enough to fly. Fortunately, baby Red Knots—as is true with most other sandpipers—can forage for themselves as soon as their down is dry and they can walk, which happens within a few hours of hatching. Credit: Brad Winn, from Audubon website
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 exacerbates the risk of extinction 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.
Tagging red knots on Cape Cod. The yellow tag is a geo-locator; the lime green alphanumeric flag means it was banded in the U.S. Credit: USFWS
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.
U.S. Fish and Wildlife Service Northeast Region 