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.

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 they've doubled their weight, then they resume their flight to the Arctic to breed young of their own.

"It's one of the world's 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 they're 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. That's where red knots spend much of their time, yet it's also the area where they've been studied the least.

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. There's 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.

It's 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.

Research has concentrated in Delaware Bay because it's 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 Service's 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 knot's territory.

Scientists research the birds worldwide, but it's most difficult for them to study red knots in Canada. There, the birds spread out over vast swaths of tundra when they're 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 don't have enough information to figure it out," Breese said.

One of the most intriguing areas of research comes from scientists who've 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 what's 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 knot's 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 another's eggs, bringing some to the surface, which then are accessible to shorebirds.

"It's not just having crabs, it's 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 they're vulnerable to gulls. After they've 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 there's 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 Virginia's Eastern Shore, and Fraser studies them. There, the red knots eat surf clams and tiny blue mussels, not horseshoe crab eggs. But Fraser said he'd 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 doesn't 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 what's 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 can't, Fraser said.

"I think there's a lot of people who can't. I'm one of them," he said. "Who wants to live in a world without shorebirds? I don't."