Every spring for centuries, red knots have fueled their 9,000-mile migration by gorging themselves on horseshoe-crab eggs at Delaware Bay. Now overzealous crab fishermen have nearly stripped the knots' cupboard bare.
By Jon R. Luoma
From behind the dunes on Delaware bay, you can hear them, red knots flying low, just above the tide line. On this sunny spring day, so many knots are flying together, coursing up and down the beach, that their wings sound like a giant deck of cards in mid-shuffle.
If you walk beyond the dunes, you can see the wing-whirring flocks, clouds of these shorebirds the size of robins: flashes of gray, black, white, maybe a bit of salmon red. A casual observer might not realize that this is one of nature's most spectacular phenomena. After all, Delaware Bay is hardly some remote nature park. Dividing southern New Jersey from the Delmarva Peninsula, the bay lies just a few hours' drive from the asphalt and smog of Philadelphia and New York, and it's a major shipping lane, with container ships and oil tankers chugging by.
Yet a closer look hints that something peculiar and remarkable is under way here. On a good day there can be thousands of small birds concentrated on a single stretch of the beach--a moving, bobbing, wavering blanket of birds, virtually obscuring the sands like a great feathered super-organism. And crawling slowly among the birds or washing up the beach on the surf are wet, brown helmet-shaped creatures that look like something from an alien world: horseshoe crabs, among the earth's most ancient animals.
Each year, from late May to early June, horseshoe crabs pull themselves onto the beaches of Delaware Bay, primarily on the new moon and full moon tides. Scoop up some sand and you'll see what's going on: The crabs are here to breed and to lay billions of tiny, pinhead-size eggs, filling the sand with tapiocalike green clumps. And precisely when the crabs begin to lay their eggs, the red knots (along with a handful of other kinds of shorebirds, including ruddy turnstones, sanderlings, and semipalmated sandpipers) arrive from points south to feed voraciously on those eggs. The two events, always simultaneous, are no coincidence. The crabs' egg-laying season and the shorebirds' migratory stopover are critically linked, a symbiosis that has evolved over thousands of years.
The knots arrive here in the course of one of the most astonishing migrations in the avian world. In terms of annual migratory mileage, they just about circumnavigate the globe. Each spring the birds fly from the southern reaches of South America to well above the Arctic Circle. Then, only weeks later, after breeding and nesting, they head back to South America. "In the world of birds," says biologist Allan Baker, "red knots are Olympic marathon runners."
Covering thousands of miles at a stretch without rest, red knots make precious few stops along that marathon springtime route. And Delaware Bay, with its rich and fatty load of horseshoe-crab eggs, might be the most important of those stops, because the birds need an abundant supply of such food to have any chance of completing their northward migration and nesting successfully.
Tragically, since the 1980s horseshoe-crab populations have been in a steep decline, a decline likely brought on by overzealous "harvesting" (mining might be a more accurate word) by enterprising commercial fishermen. Although humans don't eat horseshoe crabs, over the years untold thousands of the animals have been dredged from the ocean depths or simply retrieved from beaches by hand during their breeding season to be ground up and turned into garden fertilizer. In recent years the crabs have more often been sliced up and used as bait for eels and conch. According to a 1998 report by the Atlantic States Marine Fisheries Commission, between 1993 and 1996 the annual horseshoe-crab take more than quadrupled.
On a hot spring morning, a team of scientists waits here behind the dunes for more than two hours. As the sun reaches its zenith, an osprey works overhead. A tiny spider ardently scales the side of a human footprint in the sand. Laughing gulls cackle and shriek; red-winged blackbirds trill and rattle from the dunes. The low roar of the surf accompanies the steady whir of red knots and other shorebirds cruising the beach.
While researchers know about the horseshoe-crab decline, they're now trying to determine if, and to what extent, the red knots might also be in trouble. During the past few years a handful of scientists, accompanied by a corps of eager volunteers, have been spending a great deal of time here on Delaware Bay. They try to stay out of sight, mumbling at each other through two-way radios, swatting at mosquitoes, and waiting for a flock of shorebirds to work toward a spot on the beach called the "landing zone."
At long last, several dozen red knots walk, hop, and peck their way down the beach toward this landing zone. As a two-way radio crackles a warning, a researcher triggers a remote-control detonator, and two small cannons go off with a deep thud, launching a "capture net" that traps the birds. Moments later the red knots are taken out of the net and passed to volunteers, who sort, weigh, band, and then release the birds. I pause and am struck by the power of a red knot's tiny heart, beating against the palm of my hand.
Two weeks earlier this small bird with the furiously pumping heart and the others in the net were feeding and fattening up in the southernmost parts of South America, most of them in Tierra del Fuego, the last landfall before Antarctica. With as few as two or three stops along the way, they've flown to Delaware Bay, a journey of almost 9,000 miles. When they arrive they're at half their starting weight, their bodies devoid of fat and even some muscle. Here, in two weeks or so, each red knot will--must--double its weight. In a fit of almost maniacal eating, a knot devours more than 100,000 horseshoe-crab eggs, fueling up for the final leg of its marathon. When they've had their fill, the knots depart en masse, as suddenly as they arrived, so weighed down with new muscle and fat that they often have trouble getting airborne.
Next stop: the Arctic nesting grounds beyond Hudson Bay, 1,000 or so miles to the north. The birds arrive there in June, just as the snows melt. The nesting stop, too, will be brief: By mid-July the chicks will be born from camouflaged eggs laid on the tundra, and the females will head south again, followed in early August by the males, which have stayed behind with the growing chicks. The males, in turn, will be followed a few weeks later by the newly fledged chicks, which somehow make their way, without adults, the nearly 10,000 miles back to Patagonia or Tierra del Fuego. (In their superb book, The Flight of the Red Knot, ornithologist Brian Harrington and coauthor Charles Flowers likened the chick's early life to that of a human baby who must grow to 60 pounds in six weeks and then "stride off and emulate its mother's six-thousand-mile trans-Himalayan trek.")
The migration demands astounding navigational skills. From South America northward, for instance, the birds fly over open water, with few landmarks. A navigational error of only a few degrees would send the birds miles off course, yet they arrive at their destination here on the bay with great precision. So far, science can only provide clues to how the knots pull off this astonishing feat. From studies of other birds, it appears that migrants variously use the sun, the stars, even the length of day for navigation; many species, it seems, use traces of the mineral magnetite found in their brains and the earth's magnetic field to navigate. Radar studies show that long-distance migrants even move along the same sort of curvilinear routes that pilots plot to minimize the distance between two points on a round globe.
Scientists believe that red knots probably evolved to take on such daunting journeys largely to exploit food supplies, trading, for instance, the enormous energy spent on their northward journey for the great flush of edible insects and aquatic invertebrates in the Arctic's short summer. As they fly back south, they gorge on clams and mussels, whose populations peak in late July and August along the northeast coast of the United States. During the Argentine spring and summer, they feast on the crustaceans whose numbers explode there. Then, the following spring, they fly 9,000 miles north for the brief cornucopia of protein-rich horseshoe-crab eggs at Delaware Bay.
But here on the bay lies the rub. According to Larry Niles, chief of New Jersey's Division of Fish, Game, and Wildlife's Endangered and Non Game Species Program, the number of horseshoe crabs breeding on the New Jersey side of the bay has plummeted by 90 percent since 1990. Individual horseshoe crabs tend to bury their eggs out of reach of the red knot's inch-long bill. But when there are thousands of horseshoe crabs on a beach, their sheer creeping, crawling, digging numbers constantly churn up the sand, and eggs buried deep by one crab are exposed by the next. In short, fewer breeding crabs could not only mean far fewer eggs, it could also mean far fewer eggs the birds can get to. "They can probe for eggs to some extent," Niles says. "But that takes too much time and energy. The main advantage of Delaware Bay has been that there are so many eggs on the surface that the birds can just land and mow them, like mowing a lawn.
"We're in the middle of a drama," he says. "And it's a scary drama. If the crabs continue to fail, some red knots might not gain enough weight to make it to their breeding grounds."
Research in the past two years suggests that the birds stopping at Delaware Bay are expending more time and energy finding crab eggs while they're there. Knots outfitted with tiny radio transmitters are crossing the bay frequently, and Niles suggests that the delays in finding enough food could be extending the time the knots need to gain enough weight to reach their nesting grounds; this, in turn, could disrupt their already brief nesting period. Even if the females survive the trip to the Arctic, without sufficient food energy they could fail to produce eggs. Last summer Niles helped lead a team to the Arctic that located a cluster of nesting red knots north of Hudson Bay. Although he says the information collected from that visit remains scant and is only preliminary, it did hint that an unusual number of birds were failing to reproduce. And the most logical explanation, he thinks, is a lack of nutrition in the form of Delaware Bay horseshoe-crab eggs.
Overall, evidence suggests that red knot populations are in a steep and steady decline. In the 1980s ornithologist Brian Harrington estimated that 150,000 knots visited the bay each spring. In the past five years, counts have yielded no more than 80,000 birds. And the count in 2000 was 8 percent lower than 1999's. This past February Niles's research team returned from Tierra del Fuego's Bayhia Lomas, the red knot's most important wintering ground, with sobering news: A population that had numbered an estimated 45,000 as recently as a year ago appears to have plummeted to only 26,000, though, again, Niles cautions that the data are preliminary.
Last year, with spring's crab egg laying and shorebird gluttony still in full swing, I found biologist Allan Baker working at a table with colleague Humphrey Sitters on the bayside porch of a small beach house the researchers had rented for the season. Baker is head of the Center for Biodiversity and Conservation Biology at the Royal Ontario Museum in Toronto and one of the leading international researchers studying the plight of the red knot. He nodded as Sitters, a biologist who recently graduated from Oxford University, pointed out that the knots rest infrequently on their northward journey each spring, flying day and night literally thousands of miles, and stopping perhaps only at two or three key "stepping-stone" habitats in Argentina and Brazil before proceeding to Delaware Bay. Each stopover appears to be crucial, Sitters says. "If one goes missing, they can't get to the next one."
"It's a small chain of key sites stretched across two continents," Baker agrees. And then he nods out a large screened window at the bay just beyond the porch. "And if this bird is going to survive, this is stepping stone numero uno."
Red knots prosper in South America and the Arctic without horseshoe-crab eggs to devour. In fact, research has shown that they have the astonishing ability to reconfigure their stomachs depending on the kind of food they eat, which ranges from soft worms to small but very hard-shelled mussels that the bird crushes in its stomach. The world's two other large populations of red knots, which winter in Africa and Australia, migrate to the Arctic without the benefit of horseshoe-crab eggs, instead finding other suitable high-energy foods. Clearly, the knots along the Atlantic Flyway have evolved to take advantage of the crab eggs. It's possible that if the supply of crab eggs continues to decline, the knots would simply find other kinds of food here at Delaware Bay, although no one knows for sure. But on the porch that day, Sitters had before him a dishpan full of sand and gravel from a nearby beach. All spring he had been trying to determine what small clams or other alternative foods might be available around the bay. The prospects did not look promising.
"This sample is typical," Sitters said. "There's simply not much here. A few clams, a few snails, but nothing like what it would take to support a large population of shorebirds."
In other words, all signs continue to point to the horseshoe crab's critical role in red knot survival along the Atlantic Flyway. Finally, last August, there was some good news for red knots: The federal government established a no-catch horseshoe-crab sanctuary in and around the bay (see "New Hope for an Ancient Mariner," page 54). "Our hope is that it will stabilize the harvest," Niles says. "Unfortunately, we've already seen significant damage to the crab population, but at least this should stop further declines."
Sometime this May the first droves of horseshoe crabs will scale the beach, their shells glistening wet. There may be fewer than last year, but they will still be crawling ashore like a landing party from some alien world. On perfect, synchronized cue, the red knots will arrive, quickly packing on body weight at a miracle pace among the breeding crabs, the two species replaying like clockwork their old ecological ritual. The crabs and the knots seem to be hanging on. But with the help of some heightened human resolve, both may yet again flourish. "Red knots are fantastically interesting birds in and of themselves," Baker says. "But this association between the birds and the crabs is a wonder of the world of biology. If we can't protect this kind of heritage, then God help us."
Jon R. Luoma is a veteran Audubon contributor who lives in New Jersey, just a short drive from Delaware Bay.