If the rufa red knot flew much farther south on its migration from the Arctic, it would soon be going north again. Each fall, this little shorebird, a threatened species the size of a robin, travels the delicate thread of a path from its Arctic breeding grounds, down the entire North American East Coast, through the Caribbean, and as far south as Tierra del Fuego, Argentina, a total distance of 9,000 miles.

A favorite spot on their return trip north is the Delaware Bay, where the wealthy and powerful of Washington, DC, among others, keep beach homes along the coast. These human visitors, though, are far outclassed by the red knots, who arrive each May on their way to the Arctic, having traveled many times farther. For these birds, the Delaware Bay isn’t convenient; it’s crucial. Or at least it has been. To refuel for the rest of their trip north, red knots rely on horseshoe crabs and their eggs, but that food source is in trouble, and so are the birds.

Horseshoe crabs have been laying multitudes of eggs in the sands along the bay for millions of years. They are biologically closer to spiders than crabs, but they don’t really resemble either. They are more of an armored shell with legs and a spiky tail, and when they spawn the beach looks like it’s littered with the helmets of soldiers.

The red knots time their arrival on the bay with this spawning, gorging themselves on a buffet of the horseshoe crab’s tiny, blue-green eggs. They aren’t dainty about it. They skitter around on long legs, drilling into the sand with their beaks. The beach is covered with the hopping, flapping birds, as they compete with opportunistic shorebirds and gulls. As spectacular as the show is, it’s becoming less and less impressive. The population of horseshoe crabs, whose blue blood is important to science and medicine, is on the decline along the Atlantic coast. The population of red knots has followed.

The rufa red knot is now a federally listed threatened species. From the 1980s, their numbers have dropped by an estimated 75 to 80 percent. The US Fish and Wildlife Service and state agencies report that some years the flock counts were over 90,000 in the 1980s. They averaged around 25,000 from 2012 to 2020. “For reasons that are still being investigated, aerial counts in 2021 and 2022 were the lowest on record,” according to the report.

Biologist Larry Niles knows the trends as well as anyone. Formerly with New Jersey’s endangered species program, he now runs his own company, Wildlife Restoration Partnerships. Niles says the Delaware Bay waters were cool in 2020 and frequently disturbed by storms, which delayed the spawn. When the shorebirds came, they found no eggs. The egg buffet returned the next year, but the red knots went elsewhere. They have begun to return, but it’s hard to know what the state of the population is, Niles says. “I think we’ve hit bottom, but I don’t know.”

State agencies refer to the population as “stable.” This isn’t necessarily great, like a patient hospitalized after a car crash: not getting worse or better, but still here.

Red knots in the Delaware Bay draw photographers and scientists from around the world. Shorebird banders come from Europe and Australia, Kathleen Clark, chief of the endangered species program with New Jersey Fish and Wildlife, says. “We have many, many years of a tradition of those folks coming in to help the onsite banding teams.”

Red knots are not particularly easy to count. Clark helped with yearly practice for decades, riding in a small plane a few hundred feet up as it weaved its way around the curving coastline of the bay. “I always had a problem with motion sickness,” Clark says.

Despite such downsides, the air counts are needed. Agencies analyze the air numbers along with ground counts and data from banding to come up with a population average they trust in a ballpark way, even though counts can vary from year to year. That’s partly because red knots like to roam. In addition to flying 18,000 miles or so in a year, red knots fly for days on journeys that put long-distance truckers to shame—up to 1,500 miles, seven days straight without a rest.

They are famous for their long flights. The most famous of all is Moonbird, named for having flown farther than the distance to the moon in his long life. Moonbird, also known as B95, for his tag number, lived into his 20s and was an avian celebrity. He even has a statue in his honor at the DuPont Nature Center in Delaware. While an outlier, Moonbird wasn’t alone. Niles says most red knots live about five or six years, but he came across another recently that was at least 22 years old.

Despite this longevity, the populations are struggling. The harvest of horseshoe crabs for bait and their blood spiked dramatically in the late 1990s. Red knot populations simultaneously fell off, their long-distance flights reliant on refueling. While not all rufa red knots migrate the same distance or live in the same areas, the population that flies all the way to Tierra del Fuego and back is particularly reliant on the bay as a pit stop.

When they arrive, “they’re almost skin and bones,” says Wendy Walsh, a senior biologist in the Endangered Species Program for the US Fish and Wildlife Service. “And then two weeks later, they’re this little butterball, and they launch themselves again.”

There’s a lack of consensus on exactly how to fix the problem. New Jersey has banned all horseshoe crab harvests, while Delaware allows the harvest of males. Government agencies argue that limited, scientifically regulated harvest is not a threat to horseshoe crabs or red knots, while advocates like Niles disagree. Niles is skeptical of their statistics and argues that all harvesting of horseshoe crabs should stop until the species rebounds completely.

While horseshoe crab blood can be harvested for medical purposes in a catch and release process, some nevertheless die as a result. Niles calls the practice shameful, since there’s now a synthetic alternative.

Meanwhile, the long, fragile route of the rufa red knot contains other perils.

That includes disturbance from humans, who also love flocking to the beach, and their dogs. In some parts of Latin America, people hunt red knots. Walsh also points to habitat loss from coastal development, beach erosion from climate change, and increasingly strong storms, among other factors.

Still, there is hope. After all, a bird that can fly 18,000 miles in one year might be expected to survive many obstacles. “It was just amazing that this small creature could be so rugged,” says Philip Hoose, an author who has joined banders in the past. Holding the tiny fliers in his hand, he marveled at their resilience. “It’s a delicate thing to hold, but nature holds very few creatures as tough.”