Shorebird Migration on the Delaware Bayshore

by Jim Rapp

Naturalist Jim Rapp tells us about shorebird migration along the Delaware Bayshore.

Wildlife watchers love spectacles. Any time you find thousands of wild animals migrating together or gathering in one place, you're bound to find people watching from the sidelines with binoculars and cameras. Travelers spend a lot of money to witness great gatherings of migrating whales along our Pacific Coast, sea turtles laying eggs on tropical beaches, or thundering herds of wildebeest moving across the Serengeti.

Well, we have our own "Serengeti" right here on Delmarva along the Delaware Bayshore. One of the greatest wildlife spectacles on the planet can be observed each spring, when thousands of hungry shorebirds descend from the skies to fatten up on the energy-rich eggs of horseshoe crabs.

Many birds benefit from eating horseshoe crab eggs, but three species of migratory shorebird can be found in massive flocks swarming along the Delaware Bay beaches in May and June: Ruddy Turnstones, Semipalmated Sandpipers and Red Knots.

The Ruddy Turnstone is a stocky shorebird with a thick, dark bill that has a little upturn at the end. They have a black-and-white pattern on their face, white bellies, and bright reddish-orange sides during breeding season. Combined with their orange legs, Ruddy Turnstones have an unmistakable calico appearance. As its name implies, the turnstone can be seen flipping over stones and debris in search of food.

Semipalmated Sandpipers are smaller than Ruddy Turnstones. They have a dark bill that droops slightly at the tip, and a brownish-gray back, white belly, and dark legs. The term "semipalmated" refers to the slight webbing between their toes, which is impossible to see from a distance.

The Red Knot is a giant among the turnstones and sandpipers, and is one of our most brightly colored shorebirds. It has a short, straight bill that tapers at the end, and short dark legs. During breeding season, the knot's head and breast are reddish, with a black-and-white back that give it a salt and pepper appearance.

All three species winter along the South American coast, but the Red Knot makes one of the longest annual migrations of any bird. Some Red Knots travel more than 9,000 miles one way from Tierra del Fuego at the southern tip of South America to their summer breeding grounds in the northern Arctic. Over the course of a lifetime, a Red Knot can fly a distance that is equivalent to a trip to the moon and back.

After flying almost 5,000 miles straight to the Delaware Bayshore, Red Knots arrive emaciated and very hungry. The timing of their arrival coincides with the annual spawning of horseshoe crabs, which crawl along the Atlantic Ocean floor to the gentle shores of the Delaware Bay during big spring tides.

In warmer, southern waters, horseshoe crabs can spawn throughout the year. In the mid-Atlantic, they usually wait until the chilly water warms up to the mid 50s. Horseshoe crab spawning starts in late spring and tapers off in August, but the peak is usually around a new or full moon in late May. Eggs laid in the sand at this time are protected from the pounding surf, as the crabs are able to reach the high-tide line on the beach.

When its time to lay their tiny green eggs, thousands of large female horseshoe crabs can be seen at the waters edge coupling with smaller males. Females scratch holes in the sand and deposit their sticky eggs, which the male then fertilizes in the nest. Over the course of several nights, one female can lay more than 100,000 eggs.

This is a very good thing for hungry shorebirds. With all this activity on a narrow stretch of sand between the water and the upper beach, many eggs get kicked up to the surface.

Exposed eggs aren't likely to hatch, but they don't get wasted. Migrating shorebirds gorge themselves on up to 25,000 horseshoe crab eggs in one day. Over the course of two weeks, Red Knots can double their weight, adding 10% more mass during each day of feasting on the protein-packed eggs. This would be equivalent to a 150-pound human ballooning to 300 pounds in two weeks.

While this would not be healthy for us, it is essential for migrating shorebirds. After their Delaware Bay stopover, the chubby birds begin the second leg of their epic migration, some flying another 4,000 miles to nest and lay their own eggs on the Arctic tundra.

The Atlantic Flyway subspecies of the Red Knot has experienced more than a 75% decline since the 1980s due primarily to an increase in the commercial harvest of horseshoe crabs. In 2014, the Atlantic Flyway Red Knot was listed as threatened under the federal Endangered Species Act. Nearly 90% of the entire world's population of Atlantic Flyway Red Knot can be present along the Delaware Bay in a single day!

To protect habitat for horseshoe crabs, Red Knots, and other shorebirds, the State of Delaware launched the Delaware Bayshore Initiative in 2015. The Bayshore Initiative builds on the region's reputation as a unique and beautiful natural resource, and helps improve the shoreline economy by encouraging more locals and visitors to enjoy it through activities such as birding, fishing and boating.

Extending from Pea Patch Island near Wilmington to the City of Lewes, the Delaware Bay shoreline is widely recognized as an area of global ecological significance. Its expansive coastal marshes, shoreline, farmland and forests provide diverse habitat to many species, including migratory shorebirds.

You can explore this unique region of Delmarva along the Delaware Bayshore Byway, which extends along Route 9 from the City of New Castle to the St. Jones Neck east of Dover.

You can also explore the bayshore east of Milford, Delaware. Travel along Route 36 for about six miles and make a left at Lighthouse Road. At the end of the road, you'll spot the distinctive red roof of the DuPont Nature Center at Mispillion Harbor. This one-of-a kind site is managed by the Delaware Division of Fish & Wildlife, and includes an interpretive center and observation deck with telescopes to view the birds on the Delaware Bay shoreline. The center features exhibits about the ecological relationship between migrating shorebirds and horseshoe crabs.

After hanging out with the horseshoe crabs and shorebirds at the harbor and nearby Slaughter Beach, you can travel back to Milford for a craft beer at Mispillion River Brewing or a great meal at Abbott's Grill.

If you want to explore the Delaware Bayshore in search of early arriving shorebirds and other spring migrants, join the expert guides leading field trips during the Spring Delmarva Birding Weekend, which will be held April 21 through 24. For more information, please visit

Delmarva is fast becoming a destination for birders and nature and heritage tourists. There is no other natural spectacle on the Peninsula, or anywhere else in the world, quite like the annual arrival of egg-laying horseshoe crabs and hungry shorebirds to the Delaware Bayshore. Plan your adventure now to witness this ancient phenomenon for yourself.