Each year, red knots, sandpipers weighing no more than a coffee cup, fly a near-miraculous 19,000 miles from the tip of South America to their nesting grounds in the Arctic and back. Along the way, they double their weight by gorging on millions of tiny horseshoe crab eggs. Horseshoe crabs, ancient animals that come ashore but once a year, are vital to humans, too: their blue blood safeguards our health. Now, the rufa red knot, newly listed as threatened under the Endangered Species Act, will likely face extinction in the foreseeable future across its entire range, 40 states and 27 countries. The first United States bird listed because global warming imperils its existence, it will not be the last: the red knot is the twenty-first century’s “canary in the coal mine.” Logging thousands of miles following the knots, shivering with the birds out on the snowy tundra, tracking them down in bug-infested marshes, Cramer vividly portrays what’s at stake for millions of shorebirds and hundreds of millions of people living at the sea edge. The Narrow Edge offers an uplifting portrait of the tenacity of tiny birds and of the many people who, on the sea edge we all share, keep knots flying and offer them safe harbor.