The timing of shorebird migration is no accident. The long-distance travels of these birds have evolved over countless generations, just as the physical characteristics of a species may change over time. Year after year, the process of natural selection has refined shorebird migrations. Birds that travel efficiently and that time their travels well are rewarded with breeding success, while birds that leave too early or too late pay the price with unsuccessful reproduction -- and sometimes their lives. In this way, dozens of species of shorebirds have come to rely on the bounty the ocean provides en route between their wintering and their breeding grounds.

Eggs deposited on the beach by spawning horseshoe crabs are particularly important to shorebirds migrating along the Mid-Atlantic coast of North America. In the spring and early summer, from Maine to Mexico's Yucatan Peninsula, hundreds of thousands of horseshoe crabs move to shallow water to begin their breeding cycle. Males congregate first; females arrive shortly after, secreting a pheromone to attract mates. A male who has been successfully courted follows his mate closely as she moves ashore to lay 3,000 to 4,000 grayish-green eggs in a single shallow nest dug into the sand. The male fertilizes the eggs when the female begins to crawl away and drags him over the nest. Females may lay up to 90,000 eggs, in multiple nests and with multiple partners, each season.

If shorebirds time their migration with the peak of horseshoe crab spawning, they will double their weight during a two- to three-week stay along the Mid-Atlantic coast. They gorge on eggs, many of which are on the surface of the sand, uncovered by the spawning efforts of horseshoe crabs laying their own eggs. Scientists estimate that during a stopover, the average shorebird consumes more than 100,000 horseshoe crab eggs. Until recently, there were enough horseshoe crab eggs to meet the demand. But that's changing quickly.

The number of horseshoe crabs spawning on Mid-Atlantic beaches has declined in recent years, most likely due to the loss of spawning habitat along the Atlantic coast, and to the three-fold increase since 1990 of the use of horseshoe crabs as bait for other species. In Delaware Bay, which contains the world's largest concentration of horseshoe crabs, the number of spawning females per season has dropped from 1.2 million to 400,000 in just the last four years. Scientists agree that this alarming downturn is greatly impacting populations of shorebirds that have come to rely so heavily on horseshoe crab eggs.