Horseshoe Crab Spawning and Red Knot Migration

Every Year in May, a Must-See Delaware Phenomenon

The lives of two unlikely creatures -- an ancient "living fossil" called the Horseshoe Crab and a delicate shorebird called the Red Knot -- are intimately intertwined.

The Delaware Bay is the largest spawning area in the world for the ancient Horseshoe Crab. Every May, the Horseshoe Crabs come to Delaware Bay beaches to lay their eggs. At the same time, shorebirds -- especially the endangered Red Knot -- use the Delaware Bay beaches as a food stop on their annual migration from South America to the Arctic. They feed on the Horseshoe Crab eggs until they have enough energy for the final leg of their flight north.

Lawrence Niles, Ph.D., chief of the Endangered and Nongame Species Program of the New Jersey Division of Fish and Wildlife, says the Delaware Bay is the last stop for the Red Knots en route from their winter home in Brazil to their summer home in the Arctic. "The birds arrive at the Delaware Bay beaches at very low weights," he said, "lower than at any other stopover. They have to double their body weight,and the only nutrient they can use is Horseshoe Crab eggs."

But Horseshoe Crab populations have been declining due to over-harvesting. As a result, Red Knot populations are also declining.

The decline in Red Knot populations has chiefly been caused by the lack of sufficient eggs for foraging birds on the Delaware Bay. Prior to the over-harvest of Horseshoe Crabs, the Red Knot weight rate increase here was the highest in the world, recorded at nearly 9 grams a day in the late 1990s. The rate declined to a low of 2 grams a day in 2002. Low weight gain rates results in fewer individuals reaching the arctic breeding grounds and higher adult mortality.

Where To Observe

The ideal place to begin your visit is the DuPont Nature Center at Mispillion Harbor. This is where you will generally find the highest concentrations of Red Knots. The Nature Center features a large observation deck, as well as exhibits about Horseshoe Crabs and shorebirds.

Other reliable observation spots include Slaughter Beach, Bowers Beach, Ted Harvey Wildlife Area/Kitts Hummock, Port Mahon Road, and Fowler Beach.

For birders traveling to Delaware to view this phenomenon, the most convenient place to stay is the small town of Milford. If you'd rather stay in a larger town, Dover is just a half-hour's drive north. The resort beaches of Lewes, Dewey, and Rehoboth are a half-hour's drive south. A list of local hotels and B&Bs is available here.

The best time to observe Horseshoe Crab spawning is in the evening, at high tide, near the full and new moons of May and early June. Birds can be seen feeding at nearly any time during the day. Be sure to follow good conservation and birding etiquette. Always watch from a distance. Never disturb the Horseshoe Crabs or shorebirds in any way. Staying in your car will act as a blind and allow everybody to see the birds, without disrupting the natural processes. If you must go onto the beaches, set up during low tide and remain as quiet and still as possible. Allow the birds to come closer to you on their own.

As Chris Bennett of the Delaware Division of Parks and Recreation explains, "These birds have just traveled thousands of miles, expended incredible amounts of energy and in most cases have lost nearly half their body weight. They need to feed nearly non-stop for almost two weeks to reach the critical weight required to continue the last leg of their migration to the Arctic, stake a claim to a territory and attract a mate. Any time they are disturbed, they have to stop feeding or resting and expend much needed energy."