In May of some years, I’ve traveled to Delaware Bay beaches in New Jersey and Delaware to experience the convergence of many thousands of spawning horseshoe crabs, nesting laughing gulls, and migrating shorebirds, including red knots, ruddy turnstones, dunlin, and semi-palmated sandpipers.

Female horseshoe crabs deposit billions of tiny, dull-green eggs in the sand a few feet up the beaches. And the hundreds of laughing gulls and up to a million shorebirds congregate to consume as many of those fat-filled eggs as they can.

Those eggs fatten the shorebirds so they can complete the last lap of their trip north to the Arctic tundra to raise young. Red knots migrated the longest distance, from southern South America.    

Horseshoe crabs are not crabs but are related to scorpions and spiders. However, they have protective upper shells, as crabs do. And the 1-foot-across shells on horseshoe crabs, which have remained unchanged relics of ancient times, are shaped like horseshoes.

Pairs of horseshoe crabs creep up Delaware Bay beaches like tiny tanks to lay eggs, day or night, during the full moon or new moon in May. Each female deposits up to 100,000 eggs in the sand, which are fertilized by her mate.

Comically, several diamond-backed terrapins perch on horseshoe crabs still in shallow water. Those bay turtles, like all reptiles, are cold-blooded. They sit on the horseshoe crabs to soak up warming sunlight to have the energy to hunt food.

Meanwhile, black-headed laughing gulls, which nest in nearby saltmarshes, keep up a constant chorus of “laughing” calls while feeding on horseshoe crab eggs on Delaware Bay beaches. Being abundant and always noisy, these gulls are the icons of summer beaches and saltmarshes along the Atlantic Coast.

Hundreds of thousands of migrating shorebirds, at once on the beaches, create inspiring spectacles when ingesting horseshoe crab eggs. Their constantly moving swarms are so populous that one can’t see the sand beneath them.

Those shorebird flocks often take wing in one large mass racing over the beaches. Their airborne thousands turn this way and that in speedy flight together, as if one body. Then, suddenly, they sweep down and land on a beach like someone throwing peanuts across the sand. There they immediately consume horseshoe crab eggs again.

The convergence of migrant shorebirds, nesting gulls, and spawning horseshoe crabs can also be seen by live camera and computer screens. In your web browser, search for “live camera spawning horseshoe crabs.”       

The great convergence of horseshoe crabs and the birds that consume their eggs on Delaware Bay beaches create intriguing spectacles in May each year. And one can experience those overwhelming gatherings in person or online.


Clyde McMillan-Gamber is a retired Lancaster County Parks naturalist.

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