The Rufa red knot (a shorebird) named "Moonbird," or "B95," photographed in a crowd of birds at Fortescue, NJ. Credit: Christophe Buidin.

"Moonbird," they call him. Or sometimes, just "B95" -- the number from the band on his leg. Moonbird is the most famous, charismatic member of a group of mid-sized shorebirds called Rufa red knots, whose numbers  have plummeted so dramatically in the past several decades that they just became the first bird ever listed under the Endangered Species Act with climate change cited as a "primary threat."

Rufa red knots are among the avian world's most extreme long range flyers (especially in light of their relatively small size). They travel vast distances -- some flying over 18,000 miles -- in the course of an annual migration that begins in Tierra del Fuego, Argentina, and extends all the way up to the Canadian Arctic (and back again).

Which brings us to Moonbird's distinction: Because he is so old -- he is at least 21 -- he is believed to have flown as many as 400,000 miles in his lifetime. The distance to the moon varies, depending on where it is in its orbit, but the average distance is about 237,000 miles. Thus, Moonbird has not only flown the distance it takes to reach the moon -- he has also covered the bulk of the return voyage.

We know Moonbird's age, explains nature writer Phillip Hoose (who has written an eponymous book about him), because he was originally banded in 1995. And even then, he was an adult bird, meaning he was at least 2 years old. Since then, the same bird, with the same tag, is still being spotted, most recently in May 2014 in New Jersey. That would make Moonbird at least 21 years old, a true Methuselah for his species.

As a red knot, "if you can make it past your first year,  you’ll live to be 6 or 7," says Hoose. "The idea of a bird that’s 21, or 22, or even older, is really extraordinary."

Here's a map of Moonbird's flight path:

Assuming that Moonbird is still living -- the last sighting was in May -- there are reasons to wonder whether there will ever be another bird that is his equal. Why? Simply put, his subspecies has been devastated, and climate change will only make matters worse -- making extreme survival of the sort that Moonbird has achieved that much more difficult.

"It will become harder for a Rufa red knot to have that kind of longevity," says Hoose.

According to the Fish and Wildlife Service, there has been a 75 percent decline in numbers of Rufa red knots since the 1980s. One key reason is that the birds, during their northward migration, stop off in Delaware Bay in May and dine on the buried eggs of horseshoe crabs -- a food source upon which they vitally depend. But those crabs saw their numbers plummet when fishermen realized that if they chopped up horseshoe crabs and threw them in the water, the smell would draw in eels and conch. When numbers of horseshoe crabs crashed, so did numbers of the birds.

Horseshoe crab eggs, upon which the Rufa red knot depends for sustenance. Credit: Gregory Breese, U.S. Fish and Wildlife Service, via Flickr

"The decline started in the 90s, and there was a virtual collapse in 2003," says Larry Niles, a wildlife biologist who studies the red knot in collaboration with the American Littoral Society and the Conserve Wildlife Foundation. "We went from 50,000-60,000 birds down to 20,000, and then hit rock bottom at around 12,000."

As Niles' words suggest, the more you learn about the Rufa red knot, the more you also realize how vulnerable it is. Because these birds fly so far, their bodies change dramatically to allow them to complete their journeys. They'll take on fat and build flight muscles, and then fly 1,500 miles or more at a single stretch, arriving at their destination exhausted and very vulnerable, and overwhelmingly dependent on being able to find easy food -- which is what horseshoe crab eggs used to represent.

"We had actually attached small devices that tracked their migrations," explains Niles, "and they made 6 day flights without stopping to get to the Delaware Bay. So they would arrive completely bereft of energy, their weight would be sometimes far lower than their fat free weight, which meant they were burning muscle to get to the Delaware Bay. And the horseshoe crabs were laying eggs in such density that there was no work involved."

Niles says the "synchrony" involved here -- in which the birds managed to arrive at the Bay, from thousands of miles away, precisely when the crabs were laying eggs -- was "virtually magic." For a while, anyway. Evolution and adaptation had built a pretty good, albeit highly elaborate, survival strategy -- until everything changed.

Red knots feeding with a horseshoe crab in the foreground. Credit: Gregory Breese, U.S. Fish and Wildlife Service, via Flickr.

Clearly, for all of the reasons above, the Rufa red knot is a pretty amazing bird. Is it the most amazing of them all? That's subjective and hard to say. The Arctic tern flies farther each year (it's also a larger bird). And there's something pretty stunning about the peregrine falcon killing its prey through over 200 mile per hour dives in mid flight. Still, at minimum, the extremity and Hail Mary accuracy of the Rufa red knot's long distance flights have to win some serious respect -- and some serious concern for its plight.

The horseshoe crab population collapse, and its after-effects, is the most immediate reason for the Endangered Species Act listing of the Rufa red knot (one of six subspecies of the red knot found around the globe -- all of which are struggling, Niles says). But on top of the crash at the Delaware Bay, there are also many other changes along the birds' vast flyway, and some of those involve climate change.

The birds breed in the Arctic, the region of the world that, more than any other, is being dramatically altered by climate change. Global warming is driving more Arctic storms, suggests Niles, which can wipe out young chicks. In addition, climate changes in the Arctic are disrupting lemming populations, notes Fish and Wildlife Service biologist Wendy Walsh, which is leading to "changes to the predation pressure that the birds face when they’re on their breeding grounds." If the predators that used to dine on lemmings -- including snowy owls and a number of other birds -- can't get them, they may start to prey on red knots instead.

Meanwhile, there's also the encroachment of sea level rise on the birds' habitat, notes Walsh. As a result, the Rufa red knot has now made history -- it is the very first bird listed under the Endangered Species Act with climate change cited as a "primary" reason for the listing. Other animals that have been listed for climate reasons include the polar bear, a number of species of ice seals, and 22 species of corals that are threatened by warmer waters and ocean acidification (also caused by global warming).

This may be just the start for the listing of threatened and endangered avian species. Molly Matteson, a wildlife biologist with the Center for Biological Diversity, observes that the group has "petitioned for other bird species that have climate change as one of their threat factors. It hasn’t happened yet." According to the National Audubon Society, nearly half of U.S. bird species may be threatened by climate change in this century.

As for Moonbird? By now, says his biographer Phil Hoose, he should be back in Tierra del Fuego; we may hear a report of his arrival any day now. When it's winter here in the northern hemisphere, it's summer in the southern -- a fact that helps explain how the Rufa red knot's vast migration evolved.

"This is a real marathoner," says Hoose. "This is one of the most spectacular migrations available in bird life." For now.