IStock Photo 9396620 © Dirk Freder

The odds a person 16 or older will watch wildlife away from home in a year are 1 in 9.98. Of those, 1 in 1.99 (50%) will choose to watch shorebirds, herons, or cranes—that’s about half. Of all adults, 1 in 17.78 will bird watch in a year. And for many of these bird observers there is no better place and time to see seaside birds than Delaware in May.

Though you might not think of Delaware as a wildlife watching Mecca, every spring bird watchers flock to Delaware Bay to see one of nature’s most interesting intersections: a meeting between sundry migrating shorebirds and a host of horny horseshoe crabs.

Delaware Bay is a long estuary and outlet of the Delaware River that forms the border between Delaware and New Jersey. It’s also a critical staging site, a waypoint for exhausted migratory birds that need to rest and refuel before continuing on their 10,000-mile journey to their summer breeding and nesting grounds in the Canadian Arctic. In fact, Delaware Bay is the largest shorebird staging site in the Eastern US, with hundreds of thousands of birds visiting each spring.

The shorebirds represent more than 25 species including Ruddy Turnstones, Red Knots, Dunlins, and Sanderlings. They hail from lands as distant as Tierra del Fuego, Suriname, Patagonia, Chile, and Peru, where they spent the winter. The horseshoe crabs are ancient arthropods, living fossils with very old evolutionary histories. They are closely related to trilobites, with which they shared the Paleozoic sea. The crabs show up to mate. The birds show up to eat.

The gathering of these two animals might seem like a curious biological crossroads, but it makes perfect sense: The shorebirds need to catch up on calories after their long overseas migrations, and horseshoe crab eggs are rich in energizing fat and protein.

To prepare for the second stage of their annual odyssey—a trip that will take them another 2,000 miles to their Arctic nesting sites—the birds spend weeks feasting on the tiny, butter-colored eggs the crabs deposit in clusters along the beach. In that time, a hungry bird can more than double its body weight.

When populations of both animals are healthy, the seaside buffet works nicely. The birds eat just enough eggs to replenish themselves, but not enough to deplete the horseshoe population. However, horseshoe crab populations have been declining rapidly in the last decade as a result of overharvesting and habitat destruction. This means fewer crabs are laying eggs, which has led to a decrease in shorebird populations in Delaware Bay.