MIDDLE TOWNSHIP - Every spring, David Mizrahi sees fewer of the tiny shorebirds arriving to eat horseshoe crab eggs on the Delaware Bay. His expeditions to South America are documenting a similar decline on their wintering grounds.

But the bird is not a red knot.

Mizrahi, vice president of research and monitoring for the New Jersey Audubon Society, is tracking a shorebird called the semi-palmated sandpiper. He began studying it in 1995, when he was working toward his doctorate, and he continued his studies after being hired by the society 13 years ago.

The red knot, a state-endangered bird under consideration for federal listing, migrates from South America, stopping locally along the Delaware Bay to eat horseshoe crab eggs to gain enough weight to continue its trek to Arctic nesting grounds. A decline in its numbers, from perhaps 90,000 birds in the 1980s to just 26,000 today, gets most of the media attention.

But Mizrahi said the semi-palmated sandpiper, and several other shorebird species that depend on the crab eggs, may be declining just as fast, proportionately.

"Red knots suck all the air out of the room, but there are other shorebirds and they're declining in some cases because of what's happening on the Delaware Bay. It's apparent we're facing a decline of several species," Mizrahi said.

Most of his work is on semi-palmated sandpipers, but Mizrahi said there are also declines in sanderlings and ruddy turnstones, and he suspects several others - including dowitchers and dunlins - are down as well.

The International Union for Conservation of Nature elevated the concern status of the semi-palmated sandpiper from "least concern" to "near-threatened" in 2012. Similarly, the U.S. Fish and Wildlife Service recently ranked the semipalmated sandpiper as a "species of conservation concern."

At the height of the red knot population, before horseshoe crabs were heavily harvested on the bayshore to be used for commercial bait, there may have been 90,000 of the birds traveling each year from South America to the Arctic. Mizrahi said at that time there were as many as 3 million semi-palmated sandpipers. Now there are about 500,000.

About 50 percent of all semi-palmated sandpipers migrating north to breeding grounds are believed to stop at the Delaware Bay to feed on horseshoe crab eggs, and the numbers locally are down about 75 percent.

New Jersey Audubon has also surveyed semi-palmated wintering grounds in French Guiana, Suriname and the northern coast of Brazil since 2008 while studying surveys done previously by the Canadian Wildlife Service.

"The population is down 70 to 80 percent in wintering areas. The last two years we found the number was down in Brazil by 90 percent. We have funding to look at Venezuela and Columbia to see if they travel there," Mizrahi said.

The species nests from the eastern Canadian Arctic to Alaska, but Mizrahi said some specific nesting populations, including one that used the Hudson Bay area, has all but disappeared. Like the red knot, the bird times its migration to maximize food sources.

The question is how much of the decline can be blamed on a reduction in horseshoe crab eggs. Like red knots, the birds are not just facing changes here but also in South America, where they are still hunted, and in the Arctic, where climate change is impacting the environment.

But it is clear that fewer crab eggs are available. In the mid-1980s, egg densities could approach 50,000 eggs per square meter of sand on the Delaware Bay coast. In 2012, the density ranged from 5,700 to 8,300 eggs per square meter of sand.

"The density of crab eggs is so far below what they need. They're having difficulty meeting their needs," said Megan Tinsley, a conservation advocate with the society.

From 1995 to 1997, when Mizrahi worked on his doctorate, he said the sandpipers would feast on the eggs and gain a gram of weight per day, easily plumping up from 20 to 25 grams to the 38-45 gram range needed to make the Arctic flight. That was before overzealous crab harvesters decimated the population, which led to a ban on harvesting in 2008. There is a proposal to lift the ban, and the society opposes it.

"In the 1990s, tractor trailers were pulling in and taking the crabs out," Mizrahi noted.

But his work is not limited to this part of the bird's range.

"Humans are predators in South America and the Caribbean. No hunter targets semi-palms. They're too small. But a 20-gauge shooting a red knot or a whimbrel will hit them as well," Mizrahi said.

Coded leg bands the society's team affixed to birds during previous expeditions along with first-hand accounts provide evidence that migratory shorebirds are being hunted in Guyana, Suriname, French Guiana and throughout the Caribbean Basin. This is likely having a significant impact on viability of several species, but the degree of the impact is not fully known.

Climate change is also impacting the Arctic and possibly the relationship with predators there such as the Arctic fox and jaegers, which Mizrahi described as a predatory gull.

Mizrahi is also looking into possible contaminants in the Delaware Bay intertidal mudflats the sandpipers also feed on but red knots don't use. The last such study was done three decades ago.

Red knot numbers may have gone as low as 12,000 birds a few years ago. At that level, a single catastrophic event could spell extinction and the genetic diversity of the species can become compromised. Mizrahi has no problem with the red knots getting all the attention because other species may be "proportionately down but numerically OK."

The society wants to educate the public about the issue and especially the importance of getting more crab eggs on the beaches. The society is working on conservation measures in the wintering grounds, and knows it can't control climate change in the Arctic, but hopes to at least prevent lifting the crab harvesting ban in New Jersey.

"If we know they don't have enough food here, we can control that," Tinsley said.

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