Shorebirds and Horseshoe Crab Studies on Delaware Bay 2024 – Guts don’t Fly

Antonio Brum, a Brazilian lawyer now earning his doctorate in ecology at Unisinos University in Porto Alegre, joined us yesterday, making the same flight made by the red knot wintering in South America. This migration pattern, a crucial aspect of the red knot’s life cycle, is the focus of his research.

On April 20th, Antonio’s team in Lagoa do Peixe, Brazil, a national park south of Rio de Janeiro, caught 20 red knots and attached solar-powered satellite transmitters from the Canadian Wildlife Service and Wildlife Restoration Partnerships. They released the knots, carrying these tiny marvels that can provide hour-by-hour locations as the birds move along a perilous journey to the Canadian tundra. As seen below, the 15 birds followed different paths, some flying days across the forbidden Amazon rainforest, where a shorebird would find little comfort. Others fly across the Atlantic to the U.S. East Coast and Delaware Bay.

This map shows the locations of 15 red knot with solar powered satellite transmitters attached in Lagoa do Peixe Brazil by Brazilian biologist Antonio Brum of Unisonos University. Map by Stephanie Feigin

These birds have yet to arrive in Delaware Bay, but many have already made their way to the bay’s abundant horseshoe crabs. Our ground counts estimate at least 8000 red knots, 2000 ruddy turnstones and over 1000 sanderlings. The bay beaches are filled with dunlin, while short-billed dowitchers and semipalmated sandpipers are scattered throughout the bayshore and marsh. In the coming days, we plan to conduct a preliminary baywide boat count to estimate the number of birds inaccessible by road. This count is crucial in providing a comprehensive understanding of the migratory bird population in the area.

We made two catches during this period. Just yesterday, we filled our three-cannon net with ruddy turnstones, dunlin and sanderlings. We processed 175 turnstones and tested 100 for avian influenza, which I will describe in more detail in a future blog.

We caught 185 knots of East Point NJ on May 14, 2024. Stephanie Feigin, Alinde Fojtik, Ren Monte, and Larry Niles. Photo by Joanna Burger

The day before, Wednesday, May 14th, we made a fantastic knot catch of 175 birds, providing us with a detailed picture of the status of the flock’s condition just after their arrival in the bay. Our average weight was 120 grams, on par for the season’s first catch. Unusually, however, the distribution of weights clustered low and high. Twenty-five of the birds were under 110 grams, and seven were below 100 grams, a remarkable migratory effort for birds that were well below their fat-free weight of 125 grams. How is that possible?

Antonio Brum holds two red knots of very different weights. On the left a bird of 145 grams and on the right a bird of just 97 grams. This bird very likely absorbed it gut to pack on as much fat as possible to fly to Delaware Bay. With horseshoe crab eggs the bird can rebuild his digestive system and gain weight. Photo by Joanna Burger

If we could peer inside the birds we catch, we would witness a remarkable sight. Numerous studies, including one by a Dutch red knot biologist, Theunis Piersma, called Guts Don’t Fly,  have documented the astonishing physiological transformation of red knots and other long-distance migratory shorebirds as they prepare for journeys lasting seven days or more without stopping. They begin by building weight mostly on abundant clams in the southern hemisphere until they can carry no more and reach weights equivalent to a 125-pound person ballooning to 180 pounds. But then, to pack on more fat, the birds start to reduce the size of their digestive organs to make room for more. Our 120-pound person would now weigh more than 200 pounds and possibly up to 240 pounds. In other words, our bird would go from 125 grams to 240 grams – almost doubling its weight. This process is a testament to these birds’ incredible adaptability and resilience, a quality that remains awe-inspiring to me even after decades of study.

Birds arriving at under 100 grams, like those in the East Point catch, imply a desperate struggle to get to the bay and a likely arrival without a gut capable of grinding clams.  But they can gorge on horseshoe crab eggs, which, like all eggs, are bursting with fats and protein. The birds quickly restore their famished organs, and by the end of their stay in Delaware Bay, a short ten days or so, they rebuild their weight to about 180 grams.  It’s uncertain whether they will reduce their stomachs again because they must rely on their usual prey on arriving in the Canadian Arctic and need well-functioning stomachs to grind them into nutritious sustenance.  Even then, the fat gained from their prey gives them fuel to get to the Arctic and arrive in good condition to breed.

The videos show the conflict of safety and hunger for shorebirds on the bay. While Humphrey Sitters and I scan a flock of about 6500 red knots and 1000 ruddy turnstones mixed with other species, the flock scared itself into a frenzied retreat from the enclosed creek mouth. But once realizing the fright was unjustified they ran back, as seen in the second video.

Unfortunately, this marvel of physiological ecology flies over the heads of the regulatory agencies that pretend there is no relationship between horseshoe crabs and red knots. Incredibly, they now argue that the fishing and bleeding industries can keep grinding up crabs for bait or bleeding them to death for their blood without having any impact on the birds.

The stark reality we face today is that fishery biologists and statisticians with little field experience are making decisions that policymakers blindly follow to justify killing crabs by multinational corporations that dominate the East Coast fishery. Resisting this tide of unreality underscores the urgent need for sensible conservation efforts to stop the never-ending killing of horseshoe crabs.