In January 1985, as Guy Morrison peered out the window of a prop plane over the muddy curve of an isolated bay on the Strait of Magellan, he and his colleague Ken Ross witnessed something no other ornithologist had seen before: A flock of 7,000 rufa red knots peeling off the salt marsh, spiraling skyward like a puff of smoke. And behind them a cloud of thousands more—and thousands more beyond that.

And with that spectacle, they had solved the mystery of the wintering red knots.

Until Morrison and Ross flew over Tierra del Fuego as part of their epic trip to create an atlas of shorebirds in South America, North American birdwatchers did not know where the knots disappeared to every winter. But since they solved that riddle, another has arisen: Can a species whose migration takes it across two dozen countries and through virtually every ecosystem in the Western Hemisphere survive the ravages of climate change? The 113-gram peep’s extraordinary journey begins in late October each year, after the birds have raised their young in nests scattered across Canada’s arctic tundra. They trade their cinnamon breasts for plumage of white and grey and point their black beaks south, flying from the top of one American continent to the bottom tip of the other. That 15,000-kilometer passage, which they make in reverse each spring, counts among the longest migrations in the animal kingdom. One bird, first spotted and tagged in 1995, has now beat its 23-centimeter wings enough times to reach the moon and come halfway back.

The length of the knots’ migration has not diminished since Morrison’s discovery, but the number making the voyage has: The total population of Calidris canutus rufa abruptly dropped by 75 percent in the early 2000s. This September, after years of mulling over the decline—and spurred forward by litigation—the U.S. Fish and Wildlife Service (FWS) proposed listing the bird as a threatened species under the Endangered Species Act. The agency will publish a draft critical-habitat proposal in May and will make its final decision on the bird’s threatened status by the end of the year.

If the listing is approved, the red knot would be the first bird and third animal the U.S. has listed as threatened due primarily to climate change. (The agency listed the polar bear and wolverine last year because of climate-change related concerns.) But it wasn’t climate that decimated the knots’ population a decade ago—it was fishing. As the birds fly north each May, they make a pit stop on Delaware Bay, where they binge on horseshoe crab eggs, doubling their weight over the course of a few weeks in preparation for the next 3,000 kilometers of flight. These crab roe, once fabulously abundant in the bay, began disappearing in the late 1990s when a new fishery began harvesting them as bait for eels and conch. Meanwhile the biomedical industry became interested in the crab’s blood because it can be used to test the sterility of medical and pharmaceutical products. The horseshoe crab population plunged, and took the knots’ down with it.

In a rare spot of good environmental news wildlife managers now tentatively believe that crisis may have been forestalled, thanks to a voluntary catch restriction. But early signs of a rebound in crab and bird populations have not stemmed fears for the species’s long-term survival, explains Wendy Walsh, a biologist with the FWS. “Now that the population has already been reduced so dramatically a number of other threats would be even more likely to cause further declines,” she says. “Those threats are primarily related to climate change.”

Because the species transits across so much of the planet, climate change has many avenues with which to threaten its survival, Walsh says. In the Arctic the boom and bust cycles of lemmings have been evolving as the climate warms, so predators such as hawks and owls that once considered red knots calories of last resort now find them increasingly appetizing as the lemmings grow scarce. Ocean acidification and rising sea levels could cause problems for the shellfish the knots depend on in Tierra del Fuego and the Gulf of Mexico. Major storms, which may be hitting the U.S. east coast with more regularity and force than ever before, could cause dramatic losses if they hit at the wrong place at the wrong time.

The proposed Endangered Species Act listing has provoked 400 responses, the large majority in favor of protecting the knots. Among entities that wrote to oppose the listing are several central U.S. states and some jurisdictions along the eastern seaboard, nervous about federal restrictions on development or beach access. A petroleum lobbyist, an offshore wind-farm advocate and a Florida mosquito control district worried about their activities being constrained by the listing. Texas said it doesn’t want threatened birds threatening its economy.

But many others think the proposal should be even more rigorous—among them David Wheeler, executive director of the Conserve Wildlife Foundation of New Jersey, who argues the knot population is precarious enough to merit an “endangered” listing rather than a “threatened” one. “A single storm could potentially throw off an entire year’s migration for them, and they would take who knows how many losses,” he says. Had Hurricane Sandy arrived at the beginning of the cyclone season instead of the end it could have decimated both knot and crab populations, Wheeler says. As it was, the organization raced to artificially replace Delaware Bay’s lost sand in time for the crabs to breed there last May, he says. But FWS biologist Krishna Gifford says the endangered label is only appropriate if a species is on the brink of extinction whereas a threatened species will reach that brink without appropriate action. For now, she says, red knots fall in the latter category.

Bird lovers at the other end of the planet are watching the decision carefully. Carmen Espoz, dean of sciences at Chile’s University of Santo Tomas, has spent the last decade studying wildlife at Bahía Lomas. “It is absolutely, without a doubt necessary” that the U.S. protects the knot, she says, noting that other countries have already done so. For example, Chile successfully advocated for Bahía Lomas to be listed as a site under the Ramsar Convention on Wetlands of International Importance environmental treaty. The bay is crucial to knots and other shorebirds because its 600 square kilometers of mud and sand are filled with enough clams and mussels for the birds to feast on for months, Morrison says. “It’s worth their while to fly from one end of the hemisphere to the other for,” he says.

Morrison makes his own annual migration to the bay, where he continues the aerial surveys that he and Ross conducted in the 1980s. On that first flight in 1985 Morrison counted 41,700 birds in Bahía Lomas. This year he counted just 13,400.

For many this figure alone justifies extending legal protections for the rufa red knot. Wheeler, who helps tag the birds in Delaware Bay each May, says he is often filled with wonder at the birds’ extraordinary transglobal passage.“It’s one thing to hear about whales or some other huge creature that makes these long journeys, but this is a bird that is basically the size of a robin,” he says. “You cradle it in your hands and it feels so fragile, but then you realize it just flew in from South America. It blows the mind.”