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This Tiny Seabird’s Colossal Migration Is in Danger from All Sides

Rufa red knots rely on Delaware’s horseshoe crab–covered beaches, but climate change and development threaten both species’ survival.

© Hans Hillewaert / CC BY-SA 4.0

Like a dollop of whole-grain mustard nestled among the seashore’s rocks and shells, a clutch of horseshoe crab eggs doesn’t look all that important. But for the threatened Rufa red knot, a migratory bird about the size of a robin, these tiny packets of energy are a life preserver.

You see, by the time red knots arrive on Delaware’s beaches in May and June, the birds are little more than feathered skeletons. After a winter spent fattening up where it’s warm—anywhere from the Gulf of Mexico to Tierra del Fuego along South America’s tailbone—these mighty little fliers make a journey of several thousand miles to the Arctic, where they breed and raise chicks. This migration can exceed 9,300 miles, a distance so extreme the knots’ leg muscles and internal organs shrivel up along the way. They’d never make it without those orbs of horseshoe crab caviar.

Just one female horseshoe crab can lay more than 100,000 eggs in a single season. And while these beady-eyed ladies tuck most of their eggs below the surface of the sand, waves and other crabs often dislodge the eggs, making them available to a variety of hungry mouths.

But the Rufa red knot is especially dependent on these eggs, says Wendy Walsh, the U.S. Fish and Wildlife Service’s lead red knot biologist. For instance, she says that when horseshoe crab populations decline, as they did in the 2000s, red knot numbers also tank.

In fact, an estimated 90 percent of all Rufa red knots can be found in the Delaware Bay area come horseshoe crab spawning season. Upping the ante is the fact that the red knot population is already down to between 15,000 and 20,000 birds, compared with as many as 150,000 in the 1980s. It’s incredibly important to the future of the species that horseshoe crabs keep laying their eggs on Delaware’s beaches—and that they continue to do so in a timely fashion.

Because this whole red knot–horseshoe crab hullaballoo hinges on good timing.

If the birds show up too early, they could starve to death while waiting for the crabs to drag themselves out of the ocean’s depths. Latecomers go hungry, too. After two weeks in the surf, horseshoe crab eggs hatch and the little larvae scurry down to the intertidal flats. If tardy birds miss the bounty, they may not be able to put on enough weight to complete the journey to the tundras of Alaska and Canada—or perhaps just as costly, they’ll get there but not have enough energy to breed.

Stephanie Feigin, a wildlife ecologist with the Conserve Wildlife Foundation of New Jersey, says the bird’s success hinges on 180 grams, which is slightly heavier than a standard billiard ball. The red knots need to put on at least that much weight to get to the Arctic to do their thing.

About 80 percent of the birds Feigin and her colleagues sampled in the Delaware Bay last year met the weight threshold. And while they’re still crunching the numbers for 2017, the current feeling is that the percentage will be much, much lower.

Horseshoe crab eggs in hand, Mispillion Harbor, Delaware

Gregory Breese/USFWS

This is because it’s been a bad weather year all around. Migrating knots hit a wall of headwinds in Florida, which likely caused many of them to delay their northward migration or bow out of it altogether. In the Delaware Bay, the horseshoe crab spawn was low due to cold weather and westerly winds that hinder the crabs from emerging onto the New Jersey side of the bay. Meanwhile, up in the Arctic, the red knot breeding grounds are still under about six feet of snow, which messes with the birds’ reproductive cycle. None of these conditions would be that alarming, however, if there were still tons and tons of red knots.

“Both the red knot and horseshoe crab populations have been stable for the last few years, but they’ve been stable at a low level,” says Feigin. The buffer these species need to survive a bad storm or failed breeding years is wearing thin. Seaside development or an oil spill might be enough to send them over the edge.

And then there’s climate change. Plenty of scientific studies are already telling us that a warming world is messing up the timing of species’ interactions with each other. In Australia, mountain pygmy possums are waking up too early from hibernation and missing their yearly feast of migratory Bogong moths. In the United Kingdom, solitary mining bees now wake from their winter slumber too soon for spider orchids to get their blooms ready, disrupting the flower’s pollination scheme. And the same mixed signals are happening between winter moths and oak trees in Massachusetts.

As for the red knots, scientists aren’t totally sure yet how climate change will affect them, but some models show a negative relationship between red knot survival and high snow cover levels in the Arctic. Similarly, reports out of Tierra del Fuego reveal that in recent winters the knots have been arriving one week later than they did a decade ago.

The threat of climate-fueled sea-level rise is more clear-cut—for both the birds and the crabs. The rising sea itself will inundate the intertidal areas these species rely on, and efforts to block the ocean’s encroachment with barriers will also destroy habitat.

Walsh says the on-again, off-again relationship Americans have with the crabs is also worth watching. Starting in the 1850s, coastal farmers began grinding up horseshoe crabs—which are actually not crabs but relatives of the now-extinct trilobites—to use as fertilizer for their fields and feed for their livestock. By the 1920s upwards of two million crabs were harvested each year. Eventually, more cost-effective fertilizers and feeds gave the crabs a reprieve, but this would last only until the 1990s. That’s when Americans started hauling the animals out of the sea for use as bait in eel and whelk pot fisheries. Horseshoe crab harvests peaked in 1997, when the total take was estimated at six million pounds (about 1.5 million crabs). But again, new and better fishing technologies allowed fishermen to use less bait for each trap (instead of a whole crab for each), and the horseshoe crab hunt dropped back to more sustainable levels.

These days our uses for horseshoe crabs are a lot less wasteful and a bit more highbrow. The blood of these living fossils (which is sky-blue, by the way) has many biomedical applications. For one thing, it coagulates at the first sign of bacteria or fungi, which helps us to screen anything from vaccines and flu shots to pacemakers and artificial hips for nasty infectious agents before sending them into our bodies. Thankfully, the Atlantic States Marine Fisheries Commission monitors this industry for its impact on the overall horseshoe crab population. Around 500,000 animals are taken each year, but most are returned to the sea after a brief bleeding. It’s unclear, however, how many of the arthropods later die as a result of their “donation.” It could be as high as 15 percent.

In the end, protecting Rufa red knots (and the crabs) comes down to protecting the Delaware Bay itself, says Feigin. This will require partnerships with various stakeholders—from the fishing and aquaculture industries to medical companies to state legislators—to closely evaluate what resources we take from the bay and what the long-term consequences could be.

In the meantime, want to know two really simple things you can do right now to help benefit Rufa red knots and horseshoe crabs?

Number one: Leave shorebirds alone. Every time you (or your unleashed pup) go near a Rufa red knot, the bird will flit away from what it perceives to be a threat― a waste of valuable energy when every single kilocalorie is precious.

Two: Do the exact opposite for horseshoe crabs. In other words, if you’re taking a stroll along the sand and see a horseshoe crab lying on its backs with its tail in the air, reach down and just flip it. I know this will require some bravery, because horseshoe crabs look a bit like the face-huggers from Alien, but they don’t bite or sting, and hey, doing so might help feed a few dozen Rufa red knots.

There’s even a song you can sing while you do it. 


onEarth provides reporting and analysis about environmental science, policy, and culture. All opinions expressed are those of the authors and do not necessarily reflect the policies or positions of NRDC. Learn more or follow us on Facebook and Twitter.

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