Jon Hurdle

A sudden drop in the number of red knots visiting the beaches of Delaware Bay during migration this spring has renewed concern among scientists about the survival of the threatened shore bird’s Atlantic Coast population.

According to biologists, the number of knots that stayed to feed at the bay in May declined by about 80 percent from the same time last year. The Delaware Bay is one of the world’s most important sites for shorebird migration.

By the end of the month, only about 6,100 had been counted on the bay shores of New Jersey and Delaware, the lowest number in the almost quarter-century since the count began. The number was down sharply from 30,800 in 2019 and 32,900 in 2018, said Larry Niles, a wildlife biologist who leads the count.

The drastic reduction in migrant birds does not necessarily mean a decline in the species, Dr. Niles said. That won’t be known until after this year’s breeding season, when the birds head south again toward their wintering grounds.

But he said this season presented entirely new challenges to his team. “We must see this year as truly different than all the others,” Dr. Niles wrote in an email to volunteers who work on the count and other bird enthusiasts on May 18. “Cool weather, tropical storms and God knows what else seems to put the entire stopover in a sort of ecological lockdown.”

The knot and other shorebirds, including the semipalmated sandpiper and the ruddy turnstone, are drawn to the Delaware Bay for two or three weeks each May because of its traditional abundance of horseshoe crab eggs — essential fuel for the completion of their long-distance migrations from South America to Arctic Canada. But this year, unusually cool water temperatures for most of the month prevented many crabs from spawning, so thousands of the birds looked for food elsewhere.

About 20,000 knots were counted at the bay two days before the final tally, but they did not stay to feed because there weren’t enough eggs for the birds to eat.

“When facing beaches devoid of eggs, it’s apparent the birds choose to move on,” Dr. Niles wrote in an update for supporters on May 29. “Too bad they did so just prior to the emergence of abundant horseshoe crab eggs.”

Earlier in the month, the shortage of crab eggs on the bay beaches seemed to drive about 5,000 birds to the Atlantic coast of New Jersey, where they fed on “mussel spat,” the baby mussels that are used in aquaculture. That food, which is not as energy-rich as the crab eggs, may not allow the birds to regain enough weight to complete their migration, which must resume by the end of May to allow time for breeding in the short Arctic summer.

Just as worrying for the researchers was that, of a small sample of birds caught and weighed on May 27, 70 percent weighed less than the 180 grams they needed to fly to the Arctic and breed successfully, Dr. Niles said.

Until the last few days of May, water temperatures remained too cold for the crab-egg spawning to begin, which happens when the water reaches 59 degrees. The water usually rises to that temperature by early May, but this year, an unusual shift in the jet stream trapped cool air over the Northeast for much of the month, keeping water temperatures colder, according to David Robinson, the New Jersey state climatologist.

The Atlantic coastal population of knots, the rufa subspecies, has recently stabilized at 45,000 to 50,000 after it showed signs of crashing in the early 2000s. That decline was partly from the overharvesting of horseshoe crabs for commercial fishery bait. Demand for the crabs also comes from the biomedical industry, which uses blood from an unknown number of crabs in testing for bacterial contamination in pharmaceuticals. The industry is under pressure from conservationists to switch to a synthetic alternative.

Crab-fishing quotas, a harvest ban by New Jersey and beach closures during migration have helped prevent another decline in the bird population. But numbers are still below the level that biologists say will allow knots to survive shocks, like severe Arctic weather in their breeding grounds, new real estate development along their migration route or a sudden shortage of food at a crucial migratory stopover like the Delaware Bay.

“If they can’t find the food, then either they may cut off their migration and they won’t be able to make it up to the breeding grounds, or they make it up there in very poor condition and have a bad breeding season,” said Ken Rosenberg, an applied conservation scientist at the Cornell Lab of Ornithology in Ithaca, N.Y. “These are steeply declining birds like all the shore birds are, and if they have a year like that or a couple of years, it could be disastrous for the species.”

The federal government listed the red knot as threatened in 2014, but there’s little prospect of further government help. Even if the population drops again, Dr. Rosenberg said, dozens of environmental protections are being rolled back by the Environmental Protection Agency and other agencies.

This year, Dr. Niles’s trapping and banding team was sharply diminished by the coronavirus pandemic, which prevented dozens of volunteers from traveling. The remaining crew of eight had to spread their efforts thinly while observing strict social distancing.

Dr. Niles said he feared that the sparse migration and the shortage of crab eggs would mean that the birds might not breed successfully during the short Arctic season.

“You get to a certain date and there’s nothing that can help them, because they still have to get to the Arctic and lay eggs on a certain schedule or else their young will be lost to Arctic weather,” he said. “It’s hard to see how it’s going to work out.”

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