The threatened migratory shorebird the red knot may finally get a break, after its population fell for years as a major food source, horseshoe crab eggs, declined in the Delaware Bay.
The eggs declined due to overharvesting of the crabs and eroding of good beach habitat. There has been a moratorium on harvesting horseshoe crabs for about a decade, but the population hasn’t rebounded as hoped and the birds have continued to struggle.
But this year, the birds’ numbers and health are looking up, according to a bay-wide count from the ground and air this past week that found 34,500 of the birds along the New Jersey and Delaware bayshores.
“Last year there were only 17,000 found in the same count,” said researcher Larry Niles, a former Department of Environmental Protection scientist now associated with the Conserve Wildlife Foundation.
The robin-sized sandpipers left their wintering grounds at the southern tip of South America a few weeks ago and have stopped at the Delaware Bay to refuel before finishing their 10,000-mile flight to their breeding grounds in the Arctic. They began arriving here about May 12 and will begin leaving soon.
Counters worked from beaches and from an airplane Wednesday, timing their counts to coincide. The two numbers were compared and were close, Niles said. Of this year’s birds, 26,000 were seen on the New Jersey side from Cape May to Fortescue in Downe Township.
The bigger numbers don’t mean there are more birds this year, but that birds are staying longer and able to gain more weight as they eat horseshoe crab eggs on the bay’s beaches, Niles said. That bodes well for a successful breeding season.
The height of the horseshoe crab spawn is expected to be during Tuesday’s full moon and high tide, around the time many birds are expected to start heading for the Arctic.
“The average duration (of stay at Delaware Bay) is 12 days. They are on a pretty tight schedule,” said Mandy Dey, state Division of Endangered and Nongame Species principal zoologist. “They are able to double their body weight in 12 days, which is pretty unheard of elsewhere in the world.”
Niles said researchers have captured, weighed and released birds and found they have bulked up while here, and will leave in good condition to breed.
Last year, the bay water was cold in May, and many horseshoe crabs didn’t spawn in time for the arrival of the red knots. As a result, many birds left the bay early, underweight and probably didn’t survive to breed.
Dey said she is encouraged by the numbers, but they are still just a fraction of what they were 30 years ago, when counts found about 100,000 birds. The density of horseshoe crab eggs is also way down from then.
“Egg density has improved ... to an average of 12,000 to 15,000 per square meter, but it was 80,000 to 100,000 per square meter back in the 1980s,” she said. “They are still really just barely able to support the low number of birds we have.”
She said researchers estimate 50 to 80 percent of the hemisphere’s population of red knots and five other species of Arctic nesting shorebirds stop here for the horseshoe crab eggs.
“It’s a unique position, the last pit stop before the Arctic,” Dey said.
The other species are ruddy turnstone, sanderling, semipalmated sandpiper, dunlin and short-billed dowitcher.
“They are here to pack on weight and get away to the Arctic,” Dey said. “They want to be on (their) eggs by sometime in the first week of June.”
The incubation period is 21 days for red knots, and it takes another 21 days for hatchlings to get ready to fly south.
“They are back through in mid-July to early August,” Dey said.
The population of red knots declined substantially at one of the birds’ main wintering grounds in Chile this year, according to a group of researchers from Rutgers University-New Brunswick and other organizations. A team of scientists traveled in January to Bahia Lomas in Tierra del Fuego, Chile, and counted fewer than 10,000 of the birds. That was down from 13,000 in 2017.