The population of threatened red knots declined substantially at the birds’ 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 the red knots’ wintering grounds on Bahia Lomas in Tierra del Fuego, Chile, and counted fewer than 10,000 of the birds. That’s down from 13,000 in 2017.
The robin-sized sandpiper migrates 10,000 miles from Chile to the Canadian Arctic each spring, stopping along the Delaware Bay in large numbers to fatten up on millions of horseshoe crab eggs.
A drop in the horseshoe crab population due to overharvesting in the 1990s prompted New Jersey to institute a moratorium on harvesting them about a decade ago, but other nearby states still take them for fishing bait, and the population hasn’t rebounded in the Delaware Bay.
“Clearly, this subspecies of red knots — listed as threatened in the U.S. and endangered in Canada — is still at risk,” said Richard G. Lathrop Jr., professor of environmental monitoring in the Department of Ecology, Evolution and Natural Resources at Rutgers–New Brunswick.
The counts were done in the same way, overseen by the same person who has done them for many years — Guy Morrison of the Canadian Wildlife Service, Lathrop said.
He said the wintering grounds, like the Arctic nesting grounds, remain remote and the habitat healthy. There are no threats that would explain the population decline there. There is oil and gas drilling off the coast, but it would only provide a threat should there be a spill or other disaster.
“Because the red knot migrates 10,000 miles one way, twice a year, it creates a lot of scientific uncertainty about the cause of their decline,” said team leader Larry Niles, with the Conserve Wildlife Foundation of New Jersey.
But he said it’s clear after years of research at all their stopover points that what happens in the Delaware Bay — especially with horseshoe crabs — is the most important factor in the red knot population.
He said the horseshoe crab population has declined to about one-third of what it was decades ago.
“When the crabs declined, the birds declined,” Niles said. “Our work shows definitively the survival of red knots goes up and down depending on the resources they find on Delaware Bay,” he said.
Last year, colder water than normal in the bay delayed horseshoe crab spawning, decreasing the availability of eggs for the red knots, he said.
“Some of that (spawning) was outside the time the birds were here,” Niles said. “The reason for the delay was the temperature of the water, but ultimately it was (a problem) because there are far fewer crabs than there once were. Instead of there being a plateau of spawning for a long period of time, it’s a spike. If the spike is off, birds are in trouble.”
It made sense the wintering population had fallen from 13,000 to about 9,500, he said.
“It was a very substantial decrease,” Niles said. “We had just finished a paper saying if conditions are worsened, you should see a decline in the survival rate. Conditions worsened, the water was cool, and only 25 percent of the birds left in a good weight. That normally is about 80 percent. As a consequence, there was a lower number in Tierra del Fuego.”
Red knots are one of the world’s longest migrants. It’s about 8,000 miles from Chile to the East Coast of the U.S., and another 2,000 miles to the Arctic, where they breed, according to scientists.
Bahia Lomas is a globally significant wetlands site, according to Rutgers. It is critical for migrating and resident shorebirds, including red knots, another Arctic migrant called the Hudsonian godwit and resident Magellanic oystercatchers.
In addition to Lathrop, Niles and Morrison, the team included Stephanie Feigin and Joe Smith, from the Conserve Wildlife Foundation; Mandy Dey, from New Jersey’s Endangered & Nongame Species Program; and four other Canadians. The group was hosted by Carmen Espoz of Chile’s University of Santo Tomas, according to Rutgers.