Several beaches on the shores of Delaware Bay are rendezvous places for many thousands of spawning horseshoe crabs and up to a million migrating shorebirds of several species during the high tide of the new or full moon during the latter half of May.

It is a timed annual meeting, perfected over millennia, when shorebirds migrating to breeding grounds on the Arctic tundra stop on the beaches to get fat eating horseshoe crab eggs. The horseshoe crab gatherings and hordes of shorebirds gorging on those water creatures' tiny, green eggs create some of the world's most inspiring wildlife spectacles.

Living fossils, horseshoe crabs are aquatic arachnids that have gone mostly unchanged during the last 250 million years. They lived before and during the age of dinosaurs. They inhabit estuaries and consume mollusks and worms on the bottoms of them. The largest gatherings of them on Earth come to the sandy beaches of estuaries along the western shores of the Atlantic Ocean to spawn billions of eggs in the sand, up to 100,000 per female.

When spawning, one or a few males, which are half the size of their mates, hang onto each female. As the females crawl up the beach, laying thousands of eggs in each of a series of sandy nests, she drags along the males, which fertilize the tiny eggs in each nursery as they are pulled over them.

Large, noisy flocks of laughing gulls and about 20 kinds of northbound shorebirds crowd Delaware Bay beaches to eat horseshoe crab eggs, creating exciting natural spectacles.

The black-headed laughing gulls are the most common and obvious gull species along the Atlantic Coast in summer. They breed in nearby salt marshes and eat anything edible.

When waves from Delaware Bay wash up on the beaches, these gulls stamp on the sand to make the water carry the sand away, exposing the horseshoe crab eggs that are then easy pickings for the gulls and shorebirds.

Shorebird congregations on Delaware Bay's beaches in May are the second largest concentration of their kind in the Western Hemisphere. Hordes of shorebirds, particularly semi-palmated sandpipers, ruddy turnstones, dunlin, red knots, sanderlings, least sandpipers and short-billed dowitchers, in that arbitrary order of abundance, and other kinds in lesser numbers, throng among breeding horseshoe crabs to feast on their eggs.

Those masses of feeding shorebirds often take off in sudden flight and speed over the water, blocking the view behind them. The birds turn this way and that in perfect unison in mid-air, which shows alternating flashes of brown upperparts, then white bellies, then brown, probably to confuse predators. But soon the great flocks settle on the beaches again, like pebbles tossed across the sand, and each bird immediately begins eating.

Some shorebirds that stop along Delaware Bay beaches to refuel on horseshoe crab eggs are somewhat starved after many miles of nonstop flight for up to four days.

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Shorebirds' crab feast creates spectacle on Delaware beaches