A rare shorebird that has defied all the odds was spotted yet again last week on Delaware Bay.

He's a small thing - no bigger than a robin, weighing only as much as a stick of butter. But he has one of the longest migrations on the planet. And a lot of renown.

Scientists refer to him as B95, after the number on his leg band.

But his fans, which apparently are legion, call him the Moonbird because in his lifetime, researchers figure, he has flown the equivalent of the distance to the moon and at least halfway back.

B95 is a red knot, one of the most imperiled shorebirds now arriving on Delaware Bay.

A human-size statue celebrating his life has been erected at Mispillion Harbor in Delaware. Another is being built in Rio Grande, Argentina, where many of his kind spend the winter.

His unlikely life was even celebrated in a book, Moonbird: A Year on the Wind With the Great Survivor B95. Written by Nature Conservancy staffer Phillip Hoose, it won numerous awards and is in its third printing.

Researchers figure B95 is at least 20 years old - the oldest of his kind, as far as ornithologists know.

Each year researchers spot his distinctive leg band, they think it might be his last.

"Everybody gives him up for gone, for a ghost, every year," Hoose said. But he keeps on flying.

Red knots fly roughly 10,000 miles from the tip of South America to the Canadian Arctic, where they breed.

Each May, they stop to refuel at the Delaware Bay, where just as they arrive, horseshoe crabs are swarming ashore to lay their fat-rich eggs.

Years ago, red knots numbered nearly 100,000. But their population plummeted to about 15,000 at one point, and it has since rebounded a little.

To study the birds and learn what was happening - the harvest of crabs was eventually blamed, and restrictions were instituted - scientists began capturing the birds and banding them.

Then they followed the birds through the hemisphere to known stopping points. Scanning the flocks and noting the band colors (designating where the bird was captured) plus the number (an individual's ID), scientists could learn more about where the birds go and how long they live.

B95 kept popping up.

Last year, however, was a nail-biter. The teams of researchers from around the world who converge on the bay every May to study shorebirds had almost given up hope of seeing him when Patricia Gonzalez, an Argentine researcher, finally spotted him on May 28.

B95 still lived.

That winter, researchers looked for him in Quebec and Argentina, but no luck. They began telling themselves, "No bird lives forever."

On Thursday, a team of spotters was at Mispillion Harbor. Nigel Clark, head of projects for the British Trust for Ornithology, saw a bird with a distinctive orange band.

It said B95.

Other red knots from South America are still arriving, so maybe the intrepid old bird has learned a thing or two. "It might be an indication of its knowledge of the system and migration, having lived so long," said Kevin Kalasz, a wildlife biologist with the Delaware Division of Fish and Wildlife.

When Hoose heard the news, he was jubilant. He lives in Maine but was in Avalon, N.J., over the weekend giving talks about B95 at a shorebird festival hosted by the Wetlands Institute.

"Gosh, I'm so excited," Hoose said. "How does he do it? How does someone make his way through an ever-tightening eye of a needle . . . year after year after year?"

Worldwide, shorebird populations are in decline, mostly because of humans' effects on sites where migrants stop to fuel, like the Delaware Bay.

Now, Hoose and others figure, B95 is helping to personalize this global crisis.

"The news that B95 was seen in Delaware is a joy," said Charles Duncan, director of shorebird recovery for the Manomet Center for Conservation, in Massachusetts. "So many people have learned about shorebird conservation through this iconic little hero."

Will researchers see B95 again? On Friday, spotters on both sides of the bay were alert and eager.

Then word went out: B95 had been seen at Cooks Beach in Cape May County.

Contact Sandy Bauers

at 215-854-5147, sbauers@ phillynews.com, or follow on Twitter @sbauers. Read her blog, "GreenSpace," at www.philly.com/greenspace.