“Moonbird,” they call him. Or sometimes, just “B95” — the number from the band on his leg. Moonbird is the most famous 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 fliers (especially in light of their relatively small size). They travel vast distances — some flying more than 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 average distance to the moon 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.

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.”

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.

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. When numbers of horseshoe crabs crashed, so did numbers of the birds.

The horseshoe crab population collapse, and its aftereffects, is the most immediate reason for the Endangered Species Act listing of the Rufa red knot.

The birds also 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 says Larry Niles, a wildlife biologist, which can wipe out young chicks.

Meanwhile, there’s also the encroachment of sea level rise on the birds’ habitat, Walsh said.