Deborah Cramer is the author of Great Waters: An Atlantic Passage and Smithsonian Ocean: Our Water, Our World. She lives with her family on a salt marsh in Gloucester, MA.

Thousands of ravenous tiny shorebirds race along the water's edge of Delaware Bay, feasting on pin-sized horseshoe-crab eggs. Fueled by millions of eggs, the migrating red knots fly on. When they arrive at last in their arctic breeding grounds, they will have completed a near-miraculous 9000-mile journey that began in Tierra del Fuego. Deborah Cramer followed these knots, whose numbers have declined by 75 percent, on their extraordinary odyssey from one end of the earth to the other-from an isolated beach at the tip of South America all the way to the icy tundra.

In her firsthand account, she explores how diminishing a single stopover can compromise the birds' entire journey, and how the loss of horseshoe crabs-ancient animals that come ashore but once a year-threatens not only the survival of red knots but also human well-being: the unparalleled ability of horseshoe-crab blood to detect harmful bacteria in vaccines, medical devices, and intravenous drugs safeguards human health. Cramer offers unique insight into how, on an increasingly fragile and congested shore, the lives of red knots, horseshoe crabs, and humans are intertwined. She eloquently portrays the tenacity of small birds and the courage of many people who, bird by bird and beach by beach, keep red knots flying.