By dark of night, they howl across Delaware Bay, these winds that reach 60 miles per hour. The gales of a nor’easter — a storm that rarely happens in May — overturn everything in their path.
Along Delaware’s Pickering Beach, seaweed washes in with the tide, its long tendrils chased by a whirlwind of sand grains. A full moon lies hidden behind storm clouds. Nor’easter rains slice down like ice picks. Beach houses on Sandpiper Lane are dark; even the Red Knots resting here on their long migration north have flown. Everything from birds to humans has run for cover.
All species but one. It’s called to Pickering Beach by forces far beyond the power of the storm, beckoned by moon and tide in an age-old communion of sea and shore.
On the nights surrounding May’s full moon, magic happens. Ancient creatures — horseshoe crabs — crawl out of the ocean and onto land to mate during the highest of May’s high tides. It’s among the greatest marine spectacles on the planet.
On this spring night, the message from moon to ocean, tide to crab, is nearly interrupted by storm waves. However faint, though, the crabs can hear the call.
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The ritual has taken place for eons. Horseshoe crabs are often referred to as living fossils; similar species are found in the fossil record dating back 250 million years. Four species exist today, three in Indian and Japanese waters.
The fourth species, Limulus polyphemus, occurs along the east coast of North America from northern Maine to the Yucatán Peninsula. More than 90 percent of this population lives on the Mid-Atlantic coast; the largest concentration is in Delaware Bay. The bay’s extensive sandy beaches are one of the crabs’ top spawning spots.
With each breaking wave, horseshoe crabs, stacked atop one another like rows of coins, fight their way to shore. The clacking of shells is deafening. The crabs are bent on one goal: burying their eggs deep in the sand to ensure the next generation.
Mission accomplished, those that aren’t flipped over in the melee of wind and wave (a potential death sentence) slowly turn back toward the sea. Their return to Delaware Bay is a triumph.
Horseshoe crabs — and shorebirds like Red Knots that depend on the crabs’ eggs to fuel long-distance migrations — run a gauntlet. To survive, the crabs must avoid twin threats: fishing for eel and conch bait, and bleeding for their copper-based blue blood, which the pharmaceutical industry uses to test for bacteria in medical devices and injectable drugs.
With horseshoe crab fishery restrictions now in place, however, and a new, synthetic compound available for medical testing, smoother sailing may be ahead for crabs and birds.
Knot on the wing
Fast-forward a few years, to another date in late May. This one with calmer, sunnier weather. And a view of the action from the other side of Delaware Bay, at Sunray Beach in Villas, New Jersey, near Cape May.
It’s once again a full moon tide, and horseshoe crabs have emerged from the sea. Inch-by-inch, female crabs, often with several males attached, make their way from the bay to sands higher up the strand. There they will lay their eggs. The previous night, many did just that, leaving signs of their passage in tracks in the sand.
Horseshoe crabs bury their eggs deeper than shorebirds can reach, but waves expose large numbers of them. Red Knots scurry along Sunray Beach, poking their bills into the sand and feasting on the protein-rich special of the day: horseshoe crab eggs.
The birds, also called beach robins for their red breasts, are replenishing energy reserves before continuing their northward migration to arctic nesting grounds.
Red Knots fly more than 9,300 miles from the tip of South America to the Arctic every spring, then return every fall, making the species one of the longest-distance migrants in the animal kingdom. Biologists have identified six subspecies of Red Knot, three in the Western Hemisphere: Calidris canutus islandica, C.c. roselaari, and C.c. rufa. The last, the rufa, winters in Tierra del Fuego and breeds on the mainland and islands above the Arctic Circle.
Surviving the journey
Red Knots migrate in larger groups than most other shorebirds. They fly in segments of 1,500 miles or more, landing at stopovers called staging areas. Flocks of rufa knots, sometimes numbering in the thousands, converge on staging areas up and down the Atlantic coast, including the beaches of Delaware Bay. The knots return to the same locations year after year.
To survive their long journeys, their flight muscle mass increases and leg muscle mass decreases. Their stomachs and gizzards shrink, but their fat mass increases by more than 50 percent.
For much of the year, Red Knots eat small mollusks, shells and all. When the birds temporarily land during migration, however, they dine on fewer hard foods because of their smaller gizzards. Hence the need for soft horseshoe crab eggs.
The birds are thin when they arrive at staging areas. During their time between flights, knots eat constantly to increase their body mass, gaining around 10 percent of their weight each day.
Delaware Bay’s beaches may be the rufa’s most important rest and feeding areas. “They land here to refuel on their trek north,” says biologist Larry Niles of Niles & Smith Conservation Services in Greenwich, New Jersey. Niles is widely known for his research on Red Knots; he previously served as director of the New Jersey Department of Environmental Protection (DEP)’s Endangered and Nongame Species Program. “These birds are on a schedule,” says Niles, “one they have to stay on to reach their breeding grounds before the summertime hatching of millions of insects. All those insects are food for the knots’ young.”