PORT MAHON, Del. - Waves of wings excitedly flutter and bob over these sandy flats of Delaware Bay in an annual ecological ritual that is one of nature's greatest spectaculars.
It is a spring synchrony of migration - of shorebirds from South American winter quarters en route to Arctic breeding grounds and of arch-ancient horseshoe crabs driven from ocean bottoms to spawning beaches by tide, temperature and lunar cycles.
One million small birds, some of which fly as far as 20,000 miles a year, alight on these dunes in late May for a critical refueling stop just as the olive-brown, helmet-shaped arthropods lumber ashore to lay billions of eggs in the sand.
This frantic confluence, propelled by time-forgotten instincts, occurs for a few weeks each year on the Delaware Bay, the world's largest spawning ground of the horseshoe crab. In that time, about 50 billion of the pearly green, pinhead-size eggs will be buried in the sand to perpetuate the 350 million-year-old species.
And the ravenous migrating red knots, sanderlings, semipalmated sandpipers and ruddy turnstones - along with two dozen other shorebird species - will gorge themselves on the bounty. With the fat and protein of the crab eggs, the birds will double their weight in two weeks for the final leg to Arctic breeding grounds.
Though horseshoe eggs are not a terribly rich food, they are an abundant and reliable source. That makes these bay beaches one of the most important staging areas for shorebirds in the Western Hemisphere.
"This should be the high point for the migration," said Hugh Simmons as he led a group of Baltimore birdwatchers to the beach banquet Memorial Day weekend, when the high tide of the full moon washed a multitude of the gravid crabs onto the muddy flats.
As the single-minded birds crowded together in frenzied pecking and probing, human observers crowded the pathways with binoculars and scopes along the facing bay fronts of New Jersey and Delaware.
The horseshoes, or Limulus polyphemus - more akin to spiders than true crabs - lay more than enough eggs to survive the avian gluttony: Each female can produce 88,000 eggs a year, in four trips ashore.
Tides directed by lunar phases wash the mating pairs ashore and a month later sweep the hatched larvae out to the shallow ocean bottoms to spend their first two summers.
For the birds, the timing of the annual crab spawning is crucial to their survival. They must quickly stock up and fly north to mate and breed so their eggs will hatch in time to feast on insects of the brief arctic summer and the hatchlings grow strong enough for the fall journey back south.
Miss by a couple of weeks or find a scarcity of horseshoes here, and the birds' breeding season is lost. There's no other stopover for them between Delaware Bay and the Arctic. The birds will fail to gain enough weight to complete the final leg. Some could make it but arrive too late or fail to successfully breed. Undernourished birds that do manage to breed would lay fewer eggs.
This extraordinary congregation of far-flighted visitors has become a prime natural attraction for amateur birders along New Jersey and Delaware beaches, generating more than $10 million in ecotourism economic impact by National Audubon Society estimates.
Birders' attention helped to raise awareness of a decline in the numbers of horseshoe crabs as commercial trawlers scooped up the defenseless creatures for the growing market in bait for conch and eel fishers. The harvest of horseshoes multiplied along the mid-Atlantic, driven by rising bait prices. That prompted a horseshoe crab management plan in the region in 1995, a ban on harvest by New Jersey in 1997 and a federally enforced reduction on horseshoe crab taken two years ago.
"The bay is the keystone stopover on the migration route of the long-distance fliers," said Lawrence J. Niles, head of New Jersey's Endangered and Non-Game Species Program. "If you yank out the bay, then everybody's prediction is that the shorebird population is going to collapse."
Birds and crabs have been studied with increasing intensity over the past decade to determine the effect on the vernal phenomenon.
Thousands of the shorebirds have been trapped by cannon-fired nets, measured and weighed, and banded or outfitted with tiny radio transmitters. Biologists have stepped up the tagging of mating crabs onshore and refined surveys of horseshoes and their egg deposits.
The avian star is the red knot, which travels from Tierra del Fuego at the southern tip of Argentina to the Arctic tundra and then back again each year. The hows and whys of this migration pattern have long intrigued biologists and prompted considerable speculation.