SLAUGHTER BEACH, Del. — In years past, tens of thousands of red knots crowded the sandy beaches of Mispillion Harbor in Delaware Bay, gorging on fresh horseshoe crab eggs spawned in such abundance they turned the shoreline a gelatinous green.
Smaller than a gull but larger than a robin, the shorebirds have one of the longest-distance migrations known in the animal kingdom. Each year, the ruddy-breasted birds fly to the Canadian Arctic from their winter home in Tierra del Fuego at the tip of South America. And each year, there are fewer and fewer.
Red knots, elite athletes of the bird world, stop briefly in Delaware Bay in the spring, when the horseshoe crabs lay their eggs. They feast until theyve doubled their weight, then they resume their flight to the Arctic to breed young of their own.
"Its one of the worlds most dramatic migrants," said Nigel Clark, who heads projects for the British Trust for Ornithology. Clark, who helped set up a program in Delaware 15 years ago to tag and monitor red knots, returns on vacation from the United Kingdom each year to volunteer with the scientific teams that are counting the birds and studying why theyre in decline.
Each May, as the red knots fuel up on horseshoe crab eggs, scientists and volunteer bird-watchers from around the world also flock to Delaware Bay, to count and tag red knots and other shorebirds. Their work, which requires patience and painstaking attention to detail, will be crucial as the U.S. Fish and Wildlife Service decides later this year whether to list the red knot as a threatened or endangered species.
There are many threats that could warrant listing red knots, but climate change is one of the most challenging. As the Earth warms, the climate is changing at an accelerated pace in the polar regions. Thats where red knots spend much of their time, yet its also the area where theyve been studied the least.
"Climate change affects just about all of those stopover places, those really important places, in one way or another," said Phillip Hoose, the author of "Moonbird: A Year on the Wind with the Great Survivor B95." Hooses book follows one red knot in particular: a male tagged with a leg band in 1995 and known as "Moonbird." The nickname comes from the two decades B95 has spent flying, enough air time to make it to the moon and part of the way back.
"Hurricane Sandy accelerated the concern for this in Delaware Bay, but the times going to come where there just wont be beaches for horseshoe crabs to lay their eggs in the sand on," Hoose said. "In the Arctic, climate change could very well take away the tundra habitat that they lay their little speckled eggs in."
Red knots are beloved by bird-watchers and scientists alike, who marvel at their good looks, their stamina and the interconnected nature of their existence across a sweeping range. Theres even a statue honoring Moonbird in front of the DuPont Nature Center at Mispillion Harbor one of the best places for people to spot red knots through a scope. Researchers spotted Moonbird there again May 16.
Its difficult to know how many red knots once made up the population that stops in Delaware Bay perhaps more than 150,000, said Gregory Breese, a biologist with the Fish and Wildlife Service. But their numbers have dwindled since the mid-1990s, and there may be as few as 20,000 now.
The arrival of the red knot can be unpredictable. So when they havent shown up yet, scientists and volunteers capture and tag other, smaller shorebirds whose habits and range are similar and therefore could provide clues to changes in the red knots population. On a recent May day, they looked for ruddy turnstones that previously had been fitted with bands known as "flags." That process, known as re-sighting, allows researchers to track the larger shorebirds over time without recapturing them.
They also put new tags on smaller birds, including semipalmated sandpipers. Researchers set up small wire traps near the birds and waited for them to enter. The traps themselves are harmless, little more than a chicken-wire maze the sandpipers cant exit.
The volunteers and researchers weighed and measured the birds they trapped, and noted where the birds were tagged and when, along with other facts. Holding a wild bird is like capturing a fragile handful of downy wind for just a moment. Delicate and twitchy, its heart beats like the sweeping second hand on an old stopwatch. When freed, it doesnt look back.
Research has concentrated in Delaware Bay because its not just a crucial refueling stop, but also one of the most accessible red knot habitats to volunteers and scientists. While in the mid-Atlantic, red knots double their weight from about 3 or 4 ounces to 6 or 8 ounces in a matter of weeks.
The Fish and Wildlife Services work to decide the birds status will take into account some of the projected climate changes in the Arctic and in the South American stretch of the red knots territory.
Scientists research the birds worldwide, but its most difficult for them to study red knots in Canada. There, the birds spread out over vast swaths of tundra when theyre nesting, rather than congregating on beaches as they do in Delaware Bay in the spring.
"We know there are things that are happening outside of Delaware Bay, but we dont have enough information to figure it out," Breese said.
One of the most intriguing areas of research comes from scientists whove studied lemming cycles in the Arctic. Research released in 2008 suggested that lemmings, which breed in predictable multiyear cycles, have fallen out of that pattern.
Jim Fraser, a professor of wildlife at Virginia Tech, saw that study and thought there might be a link to the red knot. He published a paper looking at the idea that red knots are less likely to be eaten by predators such as foxes when there are lots of lemmings to eat instead, and that a dearth of lemmings might be contributing to the decline of the red knot.
The climate has become more humid in some Arctic zones, Fraser said, changing the characteristics of whats known as the subnivean layer, the area between the ground and the snowpack. That layer becomes unstable and collapses, and the habitat for the lemmings to reproduce is no longer reliable. That creates less-than-ideal conditions for big bursts of lemming populations.
The Fish and Wildlife Service will decide by the end of September whether to list red knots as endangered or threatened, agency spokesman Chris Tollefson said. Until now, their listing has been precluded by other, higher-priority listing actions. If the agency proceeds with the listing, it also will consider whether some parts of the red knot habitat are essential for red knot conservation. It could designate those places as critical habitat but only in the United States, not in the Arctic or South America.
Much of the research into the red knots decline has focused on the interplay between horseshoe crabs and the knots in Delaware Bay. The crabs were harvested for fertilizer into the early 1900s, but their modern use is as bait for eel and conch fishing; their blood also is used in biomedical testing. Catch bans and limits are in place in some areas, but the crabs numbers remain one of the key areas of concern for the red knot.
On one stretch of beach at high tide in mid-May, horseshoe crabs swarmed the waterline, eager to spawn. Females dug into the sand to bury the fertilized eggs, then they exited with the tide. Ideally, so many crabs will do the same that they churn up one anothers eggs, bringing some to the surface, which then are accessible to shorebirds.
"Its not just having crabs, its having a superabundance of crabs available, so you get lots of eggs to the surface," Breese said.
Some homeowners and visitors to the bay will walk along the beach flipping over horseshoe crabs that got stranded by the tide on their backs, where theyre vulnerable to gulls. After theyve been flipped, the crabs scuttle slowly away, back toward the sea.
Crabs, too, are tagged. Anyone who finds a tag may call in its location to the Fish and Wildlife Service. Spotters used to receive pewter horseshoe-crab lapel pins as a reward, but budget cuts ended that program.
Horseshoe crab habitat for spawning is also of concern. The Fish and Wildlife Service notes in its most recent status report on red knots that theres no adequate way to address the destruction and changes caused by erosion and rising sea level.
Habitat used by red knots for foraging in Delaware Bay is being destroyed or modified due to beach erosion, the report says. Erosion is occurring as a result of the combined effect of storms and a continued increase in sea level, and continued increases are predicted in association with global climate change."
A smaller population of red knots also stops along Virginias Eastern Shore, and Fraser studies them. There, the red knots eat surf clams and tiny blue mussels, not horseshoe crab eggs. But Fraser said hed be chagrined if his research were interpreted in a way that de-emphasized concerns about horseshoe crab populations and their spawning sites. Too much is at stake along the eastern shore of the United States, he said.
"Even if it turns out to be 100 percent about lemmings in the Arctic, it doesnt mean that conservation of shorebirds on the Delaware Bay is not absolutely crucial. Because it is one of the most important stopovers," Fraser said.
Fraser hopes to study red knots in the Arctic to get a better sense of whats happening to eggs and young birds. Other researchers would like to see more work done to study what the young birds do after their parents leave the Arctic, and where they go next, said Jean Woods, the curator of birds at the Delaware Museum of Natural History and one of the scientists who study the red knot migration each spring.
No one wants to see an end to such an epic migration, least of all the scientists who study the knots, but the climate changes in the poles are far beyond the scope of the red knot listing that might be out in a few months.
The environmentalist Aldo Leopold once wrote that there are people who can live without wild things, and people who cant, Fraser said.
"I think theres a lot of people who cant. Im one of them," he said. "Who wants to live in a world without shorebirds? I dont."