The number of red knots visiting Delaware Bay this spring plunged to a record low, pushing the shorebird’s local population closer to extinction despite a quarter-century of efforts to save it.
Naturalists scanning the beaches on both the New Jersey and Delaware sides of the bay during the May migration found only 6,880 of the birds this year, down sharply from 19,000 in 2020 and even further below the 30,000 recorded in 2018 and 2019. The latest number is now the lowest since records began in 1982.
The population of the bird’s rufa subspecies was already well below the level that would ensure its survival, and the unexpectedly sharp decline this year has renewed biologists’ fears that the numbers are now too low for the bird to survive in the long term.
“The subspecies is now much closer to extinction,” said Larry Niles, an independent wildlife biologist who has trapped, monitored and counted red knots and other shorebirds on the Delaware Bay beaches for the last 25 years.
Shocked by extent of decline
Niles said Thursday he had expected low numbers this year because of signs that last year’s migrants did not breed successfully but was shocked by the size of the latest decline.
In 2020, unusually cool ocean temperatures in the mid-Atlantic delayed the spawning of horseshoe crabs, whose eggs provide essential food for the knots to complete a migration that for some of them spans about 10,000 miles from southern Argentina to breeding grounds in Arctic Canada — one of the longest in the avian world.
Many of the red knots that arrived in the Delaware Bay last May couldn’t find food because the crabs didn’t spawn until the water warmed in early June. But by then, the birds had left in order to get to the Arctic in time to breed.
Without crab eggs to sustain them, many birds died en route to the Arctic last year, Niles said, diminishing the breeding population and leading to the sharply reduced numbers making this year’s northbound flight.
Many knots are already emaciated by the time they reach the Delaware Bay as a result of seven days of nonstop flying from South America. Without crab eggs to refuel on last year, around 40% are likely to have died en route to the Arctic, helping to explain the latest decline, Niles said.
‘A lot of them will just fall out of the sky’
“They have to fly over vast areas of unsuitable habitat like forest, and if they run out of juice, they have to either turn around or chance it until they can get to the breeding areas,” he said. “A lot of them will just fall out of the sky, and nobody would know in these vast, unpopulated areas.”
This year’s low tally may also have been driven by adverse weather in the Caribbean, making it harder for the birds to fly north. The low numbers matched those seen at two other stopover sites in South America, he said.
Numbers of other shorebird species visiting the bay this year have also fallen sharply, said David Mizrahi, vice president for research and monitoring at New Jersey Audubon. Semipalmated sandpipers, for example, are about 70% lower than a normal year, similar to the red knot’s drop, he said.
The reasons for the big declines aren’t clear, Mizrahi said, but the trend appears to be common to long-distance migrants from South America. “To see this happen so precipitously, it’s a lot of birds to be missing,” he said.
The rufa red knot, one of six subspecies globally, has a rusty red breast when in breeding plumage, and weighs less than five ounces when mature. Its dwindling numbers, and those of other shorebirds like the ruddy turnstone, have diminished the Delaware Bay’s reputation as the site of one of the world’s great shorebird migrations.
This year’s numbers are a stunning setback to years of work by Niles and his team of international shorebird experts and volunteers who have slowly helped to rebuild the population since its previous lows in the early 2000s, and turned it into an emblem for shorebird conservation. Niles’s team gathers on the bay beaches for much of May each year, trapping, weighing, measuring and banding the birds to build up a picture of the status of the population. Some of the team lobby industry and government to make changes such as New Jersey’s ban on the horseshoe crab harvest which was signed into law in 2008.
Even before the latest numbers, Niles resisted any suggestion that he had saved the rufa red knot, saying only that his efforts had arrested the earlier decline, and provided a base to rebuild the population. “We had built up some plasticity to absorb these kinds of shocks,” he said, referring to last year’s delay in horseshoe crab spawning, but the earlier uptick in the population now appears to have been wiped out.
Pushed to the brink of extinction
This year, there were plenty of crab eggs so there was no reason for birds to avoid the bay on their migration flight, Niles said. But over the long term, there has been a shortage of crab eggs, and that has pushed the knots to the brink of extinction.
“Any reasonable biologist would see this as a significant threat,” he said.
The decimation of horseshoe crabs began with an overharvest in the late 1990s in response to demand for commercial fishery bait and agricultural fertilizer. Although the crab harvest is banned on the New Jersey side of the bay it continues in the other bay states of Delaware, Maryland and Virginia, albeit subject to quotas set by the Atlantic States Marine Fisheries Commission.
Now, the crabs are also harvested in unknown numbers for their blood because it contains LAL, a clotting agent used by the pharmaceutical industry to detect bacteria in new drugs, vaccines and medical equipment. Even though the crabs are returned to the ocean after being bled, Niles and his associates estimate that about 30% of them die or are unable to breed after being bled.
An effective synthetic alternative to the crab-based product hasn’t been officially endorsed by a pharmaceutical trade group that sets industry standards. And that keeps pressure on the Delaware Bay’s horseshoe crab population, naturalists say.
“People have to stop killing horseshoe crabs until they recover,” Niles said. “The bleeding companies don’t have to kill them because there’s a synthetic. They are just killing them because they are making money.”
The fight to save the rufa red knot, which was listed by the federal government as a threatened species in 2014, is also a fight to save the whole Delaware Bay ecosystem, which has been damaged by the declining horseshoe crab population, he said.
“What we’re trying to do is not just bring back the bird,” he said. “The bird represents the dysfunction in the system. This ecosystem is being robbed of its basic natural resource.”