Subspecies of Red Knot is designated as 'threatened' bird

Tagged 'Rufa' Red Knot; photo by Fish and Wildlife Service

The U.S. Fish and Wildlife Service yesterday designated the "rufa" subspecies of the Red Knot, which frequents the Delaware Bay during its 20,000-mile migration, as a "threatened" species.

Under the federal Endangered Species Act, the threatened status means the bird is likely to become endangered in the foreseeable future. Endangered means the species faces extinction.

The designation does not affect all Red Knots, only those in the rufa subspecies.

Since the 1980s, the population of the robin-sized shorebird has fallen by 75 percent in some key areas, largely due to declines in one of its primary food sources—horseshoe crab eggs in Delaware Bay, a crucial migratory stopover site.

Horseshoe crabs—considered "living fossils" because of their origin 450 million years ago—spawn on the New Jersey and Delaware beaches in May or early June, depending on the moon phase and tides. Each female will produce 80,000 eggs, which are located in dense masses near the front of her shell. She will lay four or five clutches of these eggs, timed to successive tides. The Delaware Bay is the primary spawning grounds of the crab.

Excessive harvesting of horseshoe crabs has meant less food for the rufa Red Knot. The crabs are caught and placed in bait pots to fish for conch and eels.

It is estimated that 90 percent of the entire population of the Red Knot subspecies can be present on the bay in a single day, according to the Cornell Lab of Ornithology. The birds gorge themselves on high-protein crab eggs, and nearly double their weight to prepare for the final leg of their long journey to their Arctic breeding grounds.

Rufa Red Knots usually winter in South America—some as far south as Tierra del Fuego—and fly north to breed in the Canadian Arctic. One bird, banded by biologists in 1995, has become known as Moonbird because he has flown the equivalent of a trip to the moon and back in his 21 years of migrations.

Horseshoe Crab

During their lengthy journey, the rufa subspecies must arrive at the Delaware Bay at the exact the moment when horseshoe crabs have come ashore to lay their eggs. Climate-driven mismatches in migration timing and egg laying affect the birds’ food source.

Officials in New Jersey and Delaware are trying to manage the size of horseshoe crab harvest with the goal of stabilizing the Red Knot population. New Jersey already lists the bird as endangered and has designated the bay as a protected habitat. Law enforcement guards the birds from harassment. Delaware is awaiting guidance from the federal government, the Wilmington News Journal reported today.

Rufa Red Knots also feed on clams and mussels but these are more difficult to extract during the brief time the birds feed in the bay. Crab eggs are soft and more abundant. 

The bird has pencil-thin legs and a slender beak. They are only 9 inches long, about the size of a robin. Their wings extend to 20 inches in flight.

Each May or early June birders gather along the Delaware Bay to watch for the spectacular arrival and feeding of rufa Red Knots. In the Lehigh Valley, Red Knots in general are seen infrequently, sometimes absent for several years at a time, according to Birds of the Lehigh Valley and Vicinity, a publication of the Lehigh Valley Audubon Society.