Bird Study, Delaware Bay, Delaware Bay 2007, Expeditions and Travels, Red Knot, Shorebird

A Tough Time Finding Crab Eggs Delaware Bay 2007


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There were close to thirty people waiting around for a catch of shorebirds. Beside our core team, the Southeastern Cooperative Wildlife Disease Team studying avian influenza, and Dave Mizrahi’s team working on semipalmated sandpipers, there were ten students from Dan Hernandez’s horseshoe crab egg monitoring team. We set our net on a small beach on Money Island, a tiny bayside community just up-bay from Gandy’s Beach and Fortescue. The sky was a brilliant blue, it was not unbearably hot and the flies and gnats were tolerable, unlike the day before in Fortescue . . . . . .


Larry and Clive assess a potential knot and turnstone catch on Money Island. Peter Fullagar

There the biting strawberry flies were so dense you could count hundreds flying around every person on the beach. We set our net there to try for a catch of red knots which were rapidly becoming scarce in NJ. Two weeks ago there were about 5,000, but without sufficient horseshoe crab eggs on NJ beaches they left and crossed to Mispillion Harbor in Delaware. Still, mostly we were lucky and managed to keep up our catching program. Then we lost nearly all the knots. We could still make catches of sanderlings and turnstones, but we really needed a last catch of knots. So we tried for a small catch in Fortescue but none showed. But that day we saw close to 200 on a small beach on Money Island and we were hoping for the best.

A “flock” of strawberry flies

Over the past ten years, this is how the season often ends. The birds, desperate to find eggs, move all over the bay in search of the best spots. They have to be choosey. Dan Hernandez did his Ph.D on foraging shorebirds on the Delaware Bay and he found they seek places that have more than 20,000 eggs/square meter. The average egg density on the six best beaches in NJ is now about 3,000 eggs/square meter, but there are places, like the mouths of creeks coming out to the bay, where eggs are deposited in higher densities. At Kimble’s Creek entrance last year average densities were 16,000 eggs/square meter.

Horseshoe crab eggs (tiny greenish specs) on the surface of the sand are easily available to hungry shorebirds. Alice Ewing

Ruddy Turnstone digging for eggs (foreground). Peter Fullagar

The knots need these higher egg densities because it takes about 5,000 horseshoe crab eggs for a knot to gain 1 gram of weight. In 1997, knots were gaining an average of 8 grams/day, some up to 15 grams/day – this equates to 40,000 – 75,000 eggs consumed per day! At high egg densities, knots can gain the weight necessary to get them to the Arctic on time. As you might guess, if eggs are abundant the only limit is the rate at which knots can ingest them. If eggs are less abundant birds suffer diminishing returns, and if eggs are sparse the knots are in trouble. This is a well-known ecological relationship called the functional response which describes the how an animal’s intake of food increases as the density of its prey increases. But there is a limit to how fast an animal can pickup food and swallow it — this is the point beyond which foraging rate cannot increase any further, however much food is available. At one time, most of the bay beaches had egg densities that allowed shorebirds to feed at the optimal, or fastest, rate. Now, even though we have far fewer birds, there are only a few places where this top rate of foraging can be ach


A creek mouth at Money Island. The sandy areas on either side of the creek are protected from wave action and are good places for crabs to spawn. Alice Ewing



Semipalmated sandpipers and dunlin foraging in the creek mouth. Alice Ewing

We were on Money Island, and trying to make a last catch. After a few hours, the birds started to arrive, but far fewer than the day before. At one critical point, we had about enough to make a catch of knots and turnstones. We came within seconds of firing the net but a plane flew low over the site. It scared the birds away, and before we could induce them to come back the tide had fallen far below the catching area. We had to throw in the towel.

Nesting double-crested cormorants near Money Island. Alice Ewing

Ironically, the plane was part of Jim Fraser’s team out of Virginia Tech who were tracking radio-tagged red knots outfitted with transmitters in Virginia. One of them was seconds from being caught, but it was not to be. At least not today!

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