I was recently checking on the nesting piping plovers at a beach in Eastham, something I have done a thousand times before without event. But this time would be different, as I would seemingly learn the answer to an important natural history question that has plagued me for years.
I noticed there were a few hundred migrant shorebirds feeding in the intertidal zone in a particularly noisy and frenetic way. As is typical for May, the species were those that breed in the High Arctic, on tundra generally remote from any human intrusions. The most common were Sanderlings, many showing their attractive rusty red-brown breeding plumage that we see only briefly here on the Cape. Ruddy turnstones, Black-bellied Plovers, Dunlin, and Short-billed Dowitcher were also present.
I noticed the turnstones, striking in their flaming orange and black breeding plumage, were excavating little holes, which the sanderlings and other shorebirds would then rush to and feverishly probe their bills into. I’ve seen this before on the Cape, and have suspected the birds were eating horseshoe crab eggs, but I was never able to find hard evidence.
But this time, when I dug out some of the sand from one of these feeding holes, I found what I was looking for – among the sand grains were the tiny, greenish eggs of a horseshoe crab. I took some photos and video of the feeding frenzy, and later noticed that I also happened to catch a photo of a Ruddy Turnstone with a horseshoe crab egg in its bill.
This was the smoking gun I was looking for! Apparently, Ruddy Turnstones are the one species that will dig down into the nests, so their presence allows other shorebirds to also feed on the eggs.
Some of you may be familiar with the connection between horseshoe crabs and Red Knots and other shorebirds down in Delaware Bay, where the shorebirds feed on the eggs of the millions of horseshoe crabs that spawn there each May. So many crabs are spawning on the beaches that they are digging up each other’s nests, bringing the eggs to the surface and making them available to the hungry shorebirds. For the federally threatened Red Knot, this is a crucial food resource, without which they are not able to gain enough weight to return to their High Arctic breeding grounds and successfully nest.
In fact, horseshoe crab harvest has been banned in New Jersey for many years specifically to protect this food resource for the Red Knots.
Here on the Cape, we don’t have enough horseshoe crabs left in most places for the eggs to be a significant resource for the shorebirds, so the Red Knots, turnstones, and other shorebirds mostly fly over us in spring, heading straight to the Arctic breeding grounds from Delaware Bay. The one exception to this might be Monomoy National Wildlife Refuge, where an unpublished study found horseshoe crab eggs in the stomachs of shorebirds on the refuge, which factored into their decision to ban horseshoe crab harvest at Monomoy.
Because of the harvest ban, the waters around Monomoy are full of horseshoe crabs, and the intertidal flats are a hugely important nursery ground for the juvenile crabs – one of many ways the refuge hints at the way the Cape used to be.
On the rest of the Cape, I think for now we have to be satisfied with the occasional shorebird feeding frenzy like the one I saw, where Ruddy Turnstones have dug down and struck the green gold of horseshoe crab eggs. But with better management of the crab fishery, other parts of the Cape may be able to once again host enough spawning horseshoe crabs to be a more worthwhile stop for spring migrant shorebirds, the way Delaware Bay is now. So here’s hoping that someday we will be a flyover state no more.
https://www.youtube.com/watch?v=sOFzc1LuTY8&feature=youtu.be
