Each spring, hundreds of thousands of horseshoe crabs crawl from the bottom of Delaware Bay to lay billions of pearly green eggs on the beaches. Their salty eggs provide a feast for scavenging coastal animals, but billions more are eaten by the flocks of shorebirds that stop to rest and feed each spring after flying north from their homes in South America. In recent years the horseshoe crab population has dwindled. In turn, the number of shorebirds...