Breeding Horseshoe Crabs

May’s adventure marked a return to the same general location as my February trip, but with a different target in mind. This time I was making my way to the shores of the Delaware Bay to observe breeding horseshoe crabs and the related food web in action. On the surface, that may not sound like something worth a two-hour drive, but I was lured by a mental picture of a thick blanket of horseshoe crabs covering sandy beaches while shorebirds greedily feasted. After all, up until this trip I had almost exclusively seen horseshoe crabs singly or in small groups, and post-mortem. Their otherworldly appearance fascinates me. So, I took an extra vacation day I had in my pocket, rose early, and headed for the DuPont Nature Center and Slaughter Beach.

“Come for the crabs, stay for the shorebirds,” should be the tagline. I was a little early for the peak activity – my schedule would not permit otherwise – but still there was enough for me to understand why this phenomenon has an accompanying festival and generates quite a bit of naturalist buzz. The basics are that the Delaware Bay provides optimal conditions for horseshoe crabs to breed in late Spring, supporting more of the crabs than anywhere else. Migratory shorebirds, particularly red knots and ruddy turnstones, have in turn learned to exploit the predictability of this cycle, timing their Spring migration to include a stop on the Bay’s beaches on their northward journey. These birds are following a particularly long migratory path and thus arrive often near starvation and always in need of energy. Without this food source most would not be able to complete the trip.

My first stop was at the DuPont Nature Center, which is a renowned spot for viewing shorebirds. There was a birding group who arrived around the same time I did, and the leader was kind enough to point some birds out for me and include me as a sort of de-facto member. With her help and the aid of my scope, I was able to pick out dozens of dunlin, ruddy turnstones, sanderlings, and short-billed dowitchers. There were also a smattering of willets and semipalmated sandpipers and of course hundreds of “shorebird x” birds. What was a little surprising was an apparent shortage of red knots – the poster child for this whole thing. There were some, but not the numbers I’d expected. The Nature Center staff assured us they were around, though – they reported counting hundreds in a banding project just days before. Of course the shorebirds were not alone. laughing gulls, herring gulls, and common terns shared the beach and skies with them, as did several osprey and a small flock of barn swallows. Seemingly every post supported a double-crested cormorant. I spent about an hour and a half here and could tell this was going to be a successful day.

Next up was Slaughter Beach. This was the spot where I expected not to be able to move without treading on horseshoe crabs. That would prove to be an unfounded assumption, but the crabs were abundant – living and deceased. Every few yards was a live crab, or a carapace, or a pile of discarded crab guts. Trails in the sand told the story of their journeys after being deposited ashore by the mild surf. The birds were far less plentiful here, and I got the impression that so were the crabs. The action seemed to mostly be taking place on sandbars and sheltered flats, as well as beachheads less accessible to the public. Still, I enjoyed every moment of the stroll and came upon some unexpected bonuses. For example, I was definitely not expecting the large numbers of purple martins swooping and diving over the sand. I also had to do a double-take at several skates swimming in the shallows, and at the eastern diamondback terrapins peering cautiously from the waters. At first I put the two together in my mind, thinking I was spotting sea turtles. I may have nerded out extensively before I figured out the truth… but this was OK, because I was fine with nerding out over the truth anyway.

Having already checked off the two big purposes for the trip, I still had a few hours to kill. So, recalling a great experience from a few months ago I returned to Prime Hook National Wildlife Refuge. Once again this proved a rewarding choice. This was a second great opportunity to check off some shorebirds on my year and life lists, and included the highlight of the trip: a pair of American avocets. These were a first for me and are among the coolest-looking birds I have ever seen. The thin, upcurved bill, upright posture, and white stripe just makes them look so elegant.

Prior Months of Nature

January – Bald Eagles
February – Winter Beaches
March – Tundra Swans
April – Early Spring Wildflowers