Late last year, the U.S. Fish and Wildlife Service added the red knot to its list of endangered and threatened species. Numbers of the bird have been declining precipitously – by 75 per cent – over the last 30 years. Remarkably, this little shorebird travels from one end of the earth (Tierra del Fuego) to the other (Arctic Canada) and back every year. It does this in a series of marathon nonstop flights that can exceed 4,000 miles. The red knot is a navigational, physiological and just plain endurance marvel.

With such a way of life, timing is everything. In one of nature’s more elegant bits of evolutionary symbiosis, red knots arrive exhausted and hungry in Delaware Bay just as thousands of horseshoe crabs emerge from the sea to lay millions of energy-rich eggs on its beaches. A week or so of feasting on this bonanza and the birds are ready to complete the journey to their northern breeding grounds.

Horseshoe crabs are as remarkable. Watching these living fossils come ashore in the late spring here in Maine, though not in the numbers seen around Delaware Bay, is still an awe-inspiring sight. Consider that this littoral ritual has been going on annually for 450 million years, let continents drift as they may.

This scenario inspired Deborah Cramer to follow the red knot on its annual migration, a journey she describes in “The Narrow Edge.” She starts where the birds start, on a desolate beach in the Straits of Magellan. Heading north, Cramer travels in helicopters, small planes, “by boat, train, komatik (an Inuit sled), SUV, and ATV on rides that range from exhilarating to hair-raising.” It is not lost on her that her quarry are only “fueled on fat, warmed by feathers,” and yet they can go anywhere, no matter how remote.

Or have so far. Each stop on the knot’s migration is critical. “If only a few footholds break, the entire journey is compromised.” The “narrow edge” of the title refers to the strip of land, essential to shorebirds as much as it is desired by our own species, where continents and oceans meet. It could also imply the line between survival and extinction as the odds – habitat loss, pollution, hunting – pile up against the red knot.

These Cramer catalogs in sometimes overwhelming detail. From time to time, I found it hard to see the forest for the trees. However, nice little touches lighten the narrative too. In Argentina, she finds “winners of the high school beauty contest aspire to be biologists or rangers protecting shorebirds.” Bypassing the little town of Bishop, Texas, she notes that it produces 7 million pounds of Advil and Motrin annually, products she sorely needs after a low altitude flight over Padre Island. Cabin-bound for days by weather in Hudson Bay, she and a team of biologists, “in desperation, read aloud from ‘Shades of Gray’ [sic].”

Cramer has an ear for the telling phrase. The researchers are a “distinctive species in their own right.” Any dose of a best-selling pesticide “greater than one drop of water in an Olympic-sized swimming pool” is enough to send songbird populations into decline. The glyptodont, part of Patagonia’s prehistoric fauna, was “about the size and heft of a Volkswagen Beetle.”

Not all Cramer’s time was spent in the field. On a slab of ancient rock from a museum, she chronicles the death march of a horseshoe crab in a stagnant lagoon, 150 million years ago. She hunts archives for red tide events back to Spanish colonial times. She spends a day at Massachusetts General Hospital, where technicians are using horseshoe crab blood to test radio-isotopes used to treat cancer for contamination. Having survived decimation by being used first for fertilizer, then for bait (for eels and whelks), these ancient creatures are now being over-harvested for their blood.

“Large, vibrant populations of wildlife cease to thrive long before they become extinct,” Cramer warns, and knots and horseshoe crabs continue to take multiple hits. In an unfortunate Catch-22, diminished numbers of crabs are producing about enough eggs for the diminished number of birds, which doesn’t give much leeway for more of either. Increasing storms, she notes, can disrupt the knot’s arrival in Delaware Bay when the crabs are laying their eggs. Strangely, I found no mention of the possibility that warming seas might even more fundamentally disrupt this rendezvous by making horseshoe crabs arrive earlier. In declaring the red knot threatened, the Fish and Wildlife Service emphasized that this is their first designation listing climate change as the principal threat. As Cramer concludes, “We stand together, all of us, on the edge, facing a time fraught with challenge, filled with promise.”

Thomas Urquhart is a former director of Maine Audubon and author of “For the Beauty of the Earth.”