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Heading to Canada from as far away as Argentina, red knot sandpipers stop to feast on the eggs of horseshoe crabs at Delaware Bay. (Doug Gritzmacher)

Return of the Sandpiper

Thanks to the Delaware Bay's horseshoe crabs, the tide may be turning for an imperiled shorebird

The horseshoe crabs come from the deep, summoned by the big spring tides. Plodding and clumsy, the crabs plow along the continental shelf and through the silty waters of Delaware Bay, then drag themselves onto beaches to lay their eggs—with occasional detours to boat launches and coastal roads and waterfront parking lots. Easily flipped by waves or stranded by retreating surf, their bodies litter the shoreline like rusting artillery from a forgotten war. But their tails tick back and forth in the sand, like metronomes. They only look dead.

The red knots descend from the sky. Plain, stocky sandpipers, they can fly a distance equivalent to a trip to the moon and back over the course of their lifetimes. They exude a twitchy, almost manic energy. Many have come from Tierra del Fuego, at the tip of South America. After a brief rest in Brazil, they travel almost 5,000 miles straight to Delaware Bay on the way to their Arctic nesting grounds. Upon arrival in the bay, they are basically starving, their breastbones protruding from their downy red chests.

Each May, the birds and the crabs meet on the beach.

It's a vital rendezvous. The emaciated red knots, in the midst of one of the world's longest migrations, have two weeks to double their body weight for the nonstop flight over Canada's vast forests to the polar tundra where they breed. Luckily, Delaware Bay's beaches are the site of the world's largest horseshoe crab spawning, which has historically generated a superabundance of fat- and protein-packed eggs.

Whole conga lines of crabs assemble at high tide, females as big as dinner plates tailed by salad-plate- and saucer-size suitors. The females dig holes in the sand and deposit sticky piles of wasabi-colored specks, which the males fertilize in the nest. Because there are so many crabs, they often kick up each other's nests, bringing the eggs to the surface.

Exposed eggs are no longer viable, but they still support life. The tiny red knots gobble some 25,000 eggs a day apiece—something like a person eating 700 chicken eggs in 24 hours—and so undergo one of the most rapid weight gains of all animals. By the end of their fortnight layover, some birds achieve outright corpulence, their breasts swaying pendulously with every step. When the restless flocks finally sound their departure calls in late May or early June, the fattest individuals sometimes struggle to achieve lift-off.

The Delaware estuary is considered one of the most important shorebird stopover zones in the world, largely because of the supply of horseshoe crab eggs. But lately red knots and other migratory species have been having trouble gaining even the minimum weight necessary to continue their journeys. Since the mid-1980s, the number of visiting red knots, once close to 100,000, has fallen by more than 75 percent, to fewer than 25,000.

Scientists blame the decline on the harvest of horseshoe crabs for fishing bait, a practice that increased an estimated twenty-fold in the 1990s. States along the Atlantic Coast have since halted or restricted the harvest, and the crabs are now showing preliminary signs of recovery. For the birds, it's not as clear. In 2003, the knots were so thin they simply lacked the energy to reach the Arctic; they stopped in southern Canada and skipped an entire breeding cycle. About 15,000 passed through Delaware Bay that year, and the numbers haven't improved much since. Some scientists have forecast the imminent extinction of the rufa red knots (Calidris canutus rufa), the most common North American subspecies, which have historically relied on the crabs to refuel.

And so these days a third species flocks to Delaware and New Jersey's marshy shores: concerned ornithologists and birders from all over the world, armed with tagging gear, cannon nets, tide charts, trays of vegetarian lasagna to keep their strength up and jumbo bottles of wine. Each spring they pitch tents on the beach or crash in rental houses, keeping vigil via telescope.

The morning fog that enfolded wooden pilings and moored boats in Mispillion Harbor also helped to hide several dozen men and women crouched behind scraggly bushes. The stretch of sand before them gradually darkened as shorebirds—ruddy turnstones, semipalmated sandpipers and plenty of red knots—touched down and began jackhammering for eggs. The knot's distinctive call—whit-whit!—was lost in the greedy din. Suddenly spooked ("It wasn't me!" someone hissed over the two-way radio), the big flock levitated for a moment and sank back down to the beach. Feasting resumed. "Get ready, base camp," the radio crackled.

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About Abigail Tucker

A frequent contributor to Smithsonian, Abigail Tucker is writing a book about the house cat.

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