Then: Boom! A miniature cannon on the beach spewed a net as big as a swimming pool cover over the birds, which, now trapped, bounced like microwave popcorn in a bag. The birders, most of whom were wearing unwieldy waders, stormed out of the mist and fell to their knees in the sand to start sorting the several hundred captured knots and other shorebirds, placing them in burlap-covered boxes, which they hustled up the beach. There, volunteers sat in circles of folding chairs, passing the birds around briskly: every second in captivity was costing the birds precious calories. The flocks had arrived en masse 11 days earlier and would be leaving for the Arctic soon.
"More knot, I need more knot!" a British ornithologist called out when her box was empty. Everyone, including (understandably) the birds, was quite frantic, and I was soon recruited to help. In my hands the knots felt frighteningly light, as though they had soda straws for bones. Imagining them flying from Tierra del Fuego seemed almost silly, like picturing a paper airplane reaching the stratosphere.
Some workers plucked breast feathers (for determining a bird's sex) and bits of wing feathers (chemical analysis can reveal where it spent the winter); others measured beak lengths or affixed colored tags. The legs of many knots were already bangled with multiple tags from ornithologists in other parts of the world: red for Chile, orange for Argentina, blue for Brazil and green for the United States. (Though all rufa red knots breed in Canada, they winter along the eastern coast of the Americas.) Most coveted of all were birds whose coded tags showed that they had been caught in Delaware just a few days earlier. These birds were inserted gently into tubes resembling toilet paper rolls, to keep them still, and placed on scales, to see exactly how many grams they'd gained. The rest of the birds were also weighed, to get a sense of how the flocks were faring. Many knots had arrived weighing less than 100 grams. Ideally they would weigh at least 180 grams by the time they left.
"Oh, I've got a really skinny one," said volunteer Richard du Feu softly, examining the scrawny knot in his hand. "This one almost certainly won't make it to the breeding grounds."
Yet many people expressed some optimism about the 2009 season. They've learned, after more than a decade of disappointing migrations, not to count their knots before they hatch, but the team had estimated an impressive 25,000 birds in a survey the night before—many more than usual. It's possible that some red knots shifted their migration route in past years to scrounge for food sources beyond the bay. If so, some of those birds may be back, perhaps because there has been mild weather and crab eggs aplenty so far. Then again, last year's spawning had seemed bountiful, too, until a Mother's Day storm cooled the water and the crabs stopped laying. "Birds arriving in the middle of May expecting crab eggs didn't get any," said Kevin Kalasz, who oversees the state-run Delaware Shorebird Project. The knots left for the Arctic a week late, still disastrously underfed.
As the captive birds were released one by one to resume pecking in the sand, the tide eased in and horseshoe crabs approached the shore, great masses of them turning like gears in the shallows. Soon there were so many crabs you could hear them moving through the water, a sound like a slow boil.
Delaware Bay's diminutive waves, temperate water and sandy beaches make it a haven for horseshoe crabs (Limulus polyphemus). They are rare outside of North America's Atlantic Coast, although there are other horseshoe crab species in Asia. The creatures predate the dinosaurs and, as part of a group called chelicerates, are closer cousins to spiders than to true crabs, which are crustaceans. Nigel Clark, a research scientist with the British Trust for Ornithology, whose T-shirt sported a glow-in-the-dark horseshoe crab, called their spring spawning "one of the world's great spectacles." Crabs in more southern waters spawn throughout the year, but in the chilly mid-Atlantic they are sluggish until the ocean starts to warm. When it reaches 56 degrees, it's time to storm the beaches. The spawning lasts from late spring into August, but the high point is the last new or full moon of May, when the tides are strongest; eggs laid at the high waterline are more protected from the surf.
Locals used to grind up horseshoe crabs for fertilizer or livestock feed. The crabs' commercial value rose in the early 1970s, when scientists realized that an extract of their copper-based blood, which is a lovely cornflower blue, could be used to test for toxins in injectable drugs and in medical devices such as pacemakers and syringes. (It contains a clotting agent that is hypersensitive to toxic bacteria.) Several companies currently run labs where captured crabs are bled of roughly a third of their blood, then sent back into the sea. Most crabs reportedly recover within a week, though some environmentalists claim that the mortality is likely higher than the 7 percent to 15 percent the companies estimate.
Then, in the 1980s, with stricter regulations on other catches, Delaware Bay fishermen entered the emerging Asian market for conch and eel. Horseshoe crabs, chopped and frozen, particularly the big, spawning-age females, made great bait. By the mid-1990s, nearly three million crabs were being caught each year along the Atlantic Coast. There were still a lot of crabs left—but not the critical density needed to kick up each other's nests, making eggs accessible to knots and other birds.
Largely out of concern for the birds, the Atlantic States Marine Fisheries Commission issued its first horseshoe crab harvest restrictions in 1998, and regulations have steadily tightened since. New Jersey currently has a moratorium on harvesting horseshoe crabs. Delaware limits the catch to 100,000 males taken outside the knot-migration season, and Maryland and Virginia have also cut back. Fishermen are finding ways to use fewer crabs as bait, and researchers are trying to develop an artificial bait to replace the crabs entirely.