Shorebirds' Fate Hinges on Horseshoe Crabs

Along the coast of Delaware Bay, thousands of migratory birds depend on an ancient annual rite besieged by modern threats

  • Laura Tangley
  • Conservation
  • Sep 30, 2013

THE CALENDAR SAID IT WAS MEMORIAL DAY WEEKEND, but at southern New Jersey’s Cook’s Beach, the weather was not what you’d expect on this traditional start to summer: cold and rainy, with an onshore wind so strong that ferries crossing the bay to Delaware had been cancelled until the following day. Still, about a dozen visitors, bundled up and hunkered down over spotting scopes, had made a trip to Cook’s this weekend. Their reward: seeing thousands of shorebirds gorging on eggs laid by horseshoe crabs during the previous night’s high tide. The feast fuels the birds’ journey north to Arctic breeding grounds, some flying from as far as the southern tip of South America. One of the planet’s great natural spectacles, this annual gathering of crabs and birds is a rite of spring that has been occurring here for millennia.

But this year, some feared it might not happen at all. Last October, when Superstorm Sandy slammed into the Mid-Atlantic region, the hurricane’s fierce winds, waves and storm surges scoured Cook’s and other Delaware Bay beaches of the sand horseshoe crabs need to spawn, exposing sod banks, mud flats and piles of concrete and other rubble. “If the substrate is not right, there’s a good chance horseshoe crabs will not spawn,” says Jean Lynch, stewardship project director for New Jersey Audubon, an NWF affiliate.

An aerial survey conducted two months after the storm revealed that Hurricane Sandy destroyed 70 percent of the state’s prime horseshoe crab habitat. With less than six months before the birds would arrive, “we were really worried,” recalls survey participant Larry Niles, a biologist for the Conserve Wildlife Foundation of New Jersey and American Littoral Society who has studied shorebirds for more than 30 years. “It was a disaster. There was nothing on the beaches but rubble and muck.”

Luckily, Niles, Lynch and other environmentalists quickly came to the rescue. Raising $1.4 million from state, federal and private sources—including substantial grants from the National Fish and Wildlife Foundation and Community Foundation of New Jersey—the team organized a massive effort that during the following months removed 822 tons of rubble and trucked in 39,000 tons of sand to rebuild Cook’s and four other critical spawning beaches. They finished in late April just as the first horseshoe crabs began to crawl out of the sea. The following month, the crabs—and shorebirds—arrived in earnest and performed their ancient ritual right on schedule.

Shorebird "Super Food"

Hurricane Sandy was hardly the first threat to the bay’s springtime shorebird phenomenon. Following the collapse of cod, haddock and other North Atlantic fish populations in the 1990s, many commercial fishermen turned to harvesting whelk and eel for overseas and domestic Asian markets. They realized that horseshoe crabs made an easy and inexpensive bait for their new traps—and that the quickest way to get a lot of crabs was to head to Delaware Bay in May. (Horseshoe crabs congregate only when they breed, and the bay hosts the world’s largest concentration of breeding crabs.) “All of a sudden, trucks with out-of-state plates started backing up to the beaches,” says Niles. Between 1992 and 1997, the horseshoe crab harvest shot up from fewer than 100,000 to more than 2.5 million crabs a year.

Biologists knew the bigger harvests would spell trouble for shorebirds, and they quickly began to intensively study the animals, supplementing annual aerial surveys conducted since 1982 with ground surveys and trapping-and-banding projects. At greatest risk were three birds that both migrate farthest and rely most heavily on Delaware Bay as a stopover: the ruddy turnstone, semipalmated sandpiper and a unique subspecies of red knot, Calidris canutus rufa (above). To get to the Arctic in time to breed, all three must consume enough horseshoe crab eggs to double their weight in about two weeks.

“Horseshoe crab eggs are like a super food,” explains biologist David Mizrahi, New Jersey Audubon’s vice-president for research and monitoring. “They’re easy to find, easy to digest, and they’re filled with fat, which is exactly what migratory birds need.” And the birds need a lot of them. For thousands of years, the key to shorebird success in Delaware Bay was the sheer number of crabs that came ashore at once, each new wave of the animals digging up and making available billions of previously buried eggs. “The birds need the right food, in the right place, at the right time—and in the right abundance,” emphasizes Niles.

It wasn’t long before scientists saw signs of trouble. By 2010, the bay’s semipalmated sandpipers “had taken a nose dive,” decreasing from about 250,000 during the mid-1980s to between 65,000 and 70,000, says Mizrahi, who has been studying the birds for the past 20 years. Peaking at 140,000 two decades ago, the number of ruddy turnstones (left) fell to just more than 18,000, while red knots plummeted from 92,000 to about 15,000. Overall, the number of shorebirds of all species stopping in Delaware Bay declined by more than two-thirds: from an estimated 1.5 million during the 1980s to about 350,000. In addition, the birds no longer gained the weight they need to complete their journeys and successfully breed once they reach the Arctic.

Migratory Marvel

Conservationists are particularly concerned about the red knot, the bay’s flagship shorebird named for the rusty breast it sports during the breeding season. Most of the Western Hemisphere’s rufa red knots migrate nearly 20,000 miles a year—from South America’s Tierra del Fuego to the Canadian Arctic and back—one of the longest annual journeys of any migratory bird.

By attaching tiny, light-sensing devices called geolocators to some of the birds, Niles and his colleagues have discovered that red knots also undertake one of the longest nonstop flights: 5,000 miles during the course of six days and nights. “For a bird the size of a robin, that’s a huge distance,” says biologist and team member Amanda Dey of the New Jersey Department of Environmental Protection’s Division of Fish and Wildlife. “That a red knot can stay in the air for six days straight without stopping is truly amazing.”

It also is physiologically stressful. When knots finally arrive in the bay, “they need to eat a lot of eggs fast,” Dey says. Unlike fall migration, when the birds have the luxury of more time to search for prey, “in spring, red knots are stressed for time if they’re going to make it to the Arctic and have a go at breeding. It’s an evolutionary strategy that has started to unravel because there no longer are enough eggs on the beaches.”

That unraveling can be seen in recent counts from the birds’ South American wintering grounds, which can be more accurate indicators than counts at stopovers, where birds continually are coming and going. Aerial surveys conducted in Patagonia and Tierra del Fuego during the mid-1980s by the Canadian Wildlife Service recorded nearly 68,000 red knots. When the same biologists surveyed the same region in 2012, they found only about 15,000 of the birds.

Armed with such alarming data, conservationists began to push hard for limits on the horseshoe crab harvest. Thanks in large part to pressure from New Jersey Audubon members, that state’s legislature in 2008 passed a moratorium on taking any crabs for bait. The ban can be lifted only if the state receives credible evidence that shorebird populations have recovered. On the other side of the bay, Delaware now follows limits set by the federal Atlantic States Marine Fisheries Commission (ASMFC): 162,136 male-only crabs per year. ASMFC also has decreased the number of Delaware Bay crabs that Maryland and Virginia can take.

Climate Wild Card

Biologists say crab harvest restrictions, while encouraging, may not be enough to recover depleted shorebird populations. For the past three springs—2011 to 2013—they report modest increases in the number of birds on the bay’s beaches. The birds also have gained the weight they need. “But having a good year on Delaware Bay does not guarantee they will succeed in the Arctic,” Mizrahi says. Moreover, populations remain such a small fraction of what they once were that red knots in particular “could just blink out if they get hit by a disaster like a bad storm in the Tropics or an oil spill in Delaware Bay,” Niles says.

An ASMFC model, to which the three scientists contributed, predicts that even with harvest limits, the bay’s horseshoe crabs will not recover for another 60 years—“probably too late for red knots,” Niles says. He and his colleagues suggest several steps that can be taken now to decrease the crabs’ recovery time and give the birds a better chance. The most important, perhaps, would be stricter regulation of horseshoe crab catch by the biomedical industry, which uses the animals’ unusual copper-based, blue blood to test drugs, vaccines and surgical implants for bacterial contamination. Collecting hundreds of thousands of Delaware Bay crabs a year, the companies are supposed to return the animals unharmed after withdrawing blood. “But we suspect they’re killing more crabs than they own up to,” Niles says, “and nobody is allowed to see their data.”

Increased spawning habitat also could help crabs recover faster, and post-Sandy restoration efforts showed that strategy is feasible. Even before the hurricane hit, Lynch and Niles were working together to try and repair Moore’s Beach, one of five targeted after the storm. Once among the most important New Jersey beaches for crabs and shorebirds, Moore’s for many years had been bypassed by most animals because it was covered with rubble left behind by abandoned beach homes. This spring, for the first time in 20 years, crabs and birds came together in large numbers on a sandy, rubble-free Moore’s Beach.

A newly recognized wild card in the race to bring back shorebirds is global warming. Because the birds require appropriate conditions in the many different habitats they use, “long-distance migrants like red knots are among the species most vulnerable to climate change,” says NWF Senior Scientist Doug Inkley, coauthor of a recent Federation report, Shifting Skies: Migratory Birds in a Warming World. One problem is that these birds spend much of their lives near the planet’s poles, places where climate change already is altering the environment dramatically. Another threat is that warmer waters could change the timing of horseshoe crabs’ temperature-sensitive spawning, putting crabs and shorebirds out of sync.

As last fall’s Hurricane Sandy made clear, warming-related increases in storm intensity, combined with sea-level rise, also can destroy beaches that both crabs and shorebirds depend on. But Niles feels less helpless about this threat than he did before the storm. “Last spring, we showed that with current technology and at a reasonable cost, we can rebuild our beaches,” he says. “And we have to do it. We can’t afford to give up on Delaware Bay.”

NWF in Action: Helping Wildlife on Delaware Bay

NWF affiliates New Jersey Audubon and Delaware Nature Society have long worked to protect horseshoe crabs (whose eggs, left, fuel shorebird migration) by engaging in research, education and advocacy. Both groups have lobbied state and federal governments to restrict crab harvests and are pressing the U.S. Fish and Wildlife Service to designate the red knot endangered. NWF works with groups in several eastern states to address impacts of sea-level rise and climate-fueled storms such as Sandy.

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Senior Editor Laura Tangley visited with shorebird scientists working in New Jersey last May.

Help NWF protect wildlife against climate change and other threats. >>


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