A Feast of Crabs

At Delaware Bay: Nature's Energy Bars for Migrating Birds
  |  Gorp.com

About fifteen years ago, on an unusually warm day in early May, I took a stroll along a wild stretch of Delaware Bay beach within Delaware's Prime Hook National Wildlife Refuge. I remember there being some shorebirds about, and a number of horseshoe crabs—rust-colored and strangely prehistoric in appearance—littered along the beach.

Neither I nor anyone else at that time—including within the naturalist community—had any idea that I walked through the epicenter of a place that annually hosts one of the most remarkable natural spectacles that can be seen in North America. Unseen by me, horseshoe crabs miles offshore in the Atlantic were responding to lengthening days and beginning to move along the ocean floor toward the Delaware Bay, where they breed on the beaches. In South America, in places like Tierra del Fuego and the Pampas, many species of wintering shorebirds were becoming restless as the Southern Hemisphere edged toward its colder months. In two weeks they would all be here, a million crabs and even more birds.

Where I'd seen a few dozen horseshoe crabs there would soon be thousands of them, marching in with the tide, making the shallows boil with their activity and piling up into layers on the sand, nearly obscuring the beach in an immense orgy of egg-laying and fertilization. And feeding on the crab eggs would be tens of thousands of birds—red knots, semipalmated sandpipers, ruddy turnstones, and sanderlings mostly; great clouds of birds swirling up and down the beach for miles, darkening the sky, the sound of beating wings and screaming birds nearly deafening. It is an awesome scene, the more so for the vast distances, expanses of evolutionary time, and delicate ecological balances it encompasses.

The horseshoe crab has some remarkable characteristics, as befits an animal that has been around for 200 million years—longer than any other on earth today. But for this story the most remarkable quality these "living fossils" have is their fecundity. A spawning female horseshoe crab may lay as many as 88,000 pinhead-size eggs each spring. Multiply this number by a million and you'll have some sense of the staggering amount of eggs laid on the Delaware Bay beaches during a few short weeks in late May. On many of the beaches during this time there can be as many as 100,000 eggs per square yard! But amazingly, only about 130,000 of all eggs laid in the bay each year will survive to become adult horseshoe crabs.

A primary reason is this: To the huge flocks of shorebirds that never fail to appear on the Delaware Bay just as the horseshoe crab spawning is peaking, the billions of protein-rich eggs are the difference between life and death, on a species-wide scale. They arrive after a 70-hour, 4,000-mile nonstop flight over open ocean from South America, exhausted, ravenous, having lost as much as half their body weight during the passage. And they face another 3,000-mile journey to reach their breeding grounds in northern Canada—they need a food source on precisely the scale the horseshoe crabs offer them to be able to complete their migration. A few weeks of nearly constant feeding interspersed with rest and they're on their way north.

It wasn't until the late 1980s that biologists realized how important a staging area the Delaware Bay is to migrating shorebirds, and how dependent they are on the horseshoe-crab population. A precipitous drop in the bay's crab population could very well wipe out entire species of birds in North America; for example, more than 80 percent of the continent's red knots stop over on the bay's beaches each year.

Various kinds of human activity place this rite of spring at risk. People have been harvesting horseshoe crabs from the Delaware Bay for more than a century—these days mostly for use as bait by eel fishermen—and continue to take them by the hundreds of thousands. A large oil spill in the bay, through which 70 percent of the oil arriving on the east coast arrives, is soberingly easy to imagine. Development of Delaware Bay beaches reduces habitat available to nesting crabs and migrating birds, and ironically, the throngs of birders who've been coming since about 1990 to see the crabs and birds commingle have become a problem—shorebirds are notoriously jumpy, and when an overeager human intrudes too closely upon them, they'll waste valuable energy and feeding time to find another place to feed away from humans.

The New Jersey and Delaware Audubon Societies have been working with state governments and other environmental organizations to protect this fragile but crucial node of our environment. Efforts to promote readiness for a possible oil spill, protect habitat, minimize disturbance of migrating birds by visitors and area residents alike, and protect and manage the bay's horseshoe-crab population are all important if the crabs and their accompanying swarms of shorebirds are going to continue to be part of our natural scene.

Published: 30 Apr 2002 | Last Updated: 15 Sep 2010
Details mentioned in this article were accurate at the time of publication


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