Red Knots: A Tiny Bird’s Massive Migration

Bird migrations defy human understanding. Every year, for example, Adélie penguins trek more than 8,000 miles following the Antarctic ice coast as it expands then contracts. This distance isn’t unusual as far as bird migrations go but the thing is, the penguins wobble all that way on foot.

Here’s another example: sooty shearwaters migrate in a circular route from islands in the southern hemisphere to the Arctic, then back to the original islands. All in all, this trip can reach 40,000 miles. A monumental flight like this done year after year seems far-fetched, but in terms of sheer distance, it gets wilder.

Mile for mile, the arctic tern’s migration is uncontested. They more or less migrate the farthest distance possible—from the Arctic to Antarctica and back—but instead of simply flying as the crow would, they chart a wandering course that adds thousands of miles to their migration, presumably all for the added challenge. The longest of these migrations exceeds 50,000 miles. For reference, a flight from Sydney to New York falls just short of 10,000 miles.

Then there are red knots (Calidris canutus), small sandpipers with toothpick legs, a stocky bill, and beautiful rusty-red breeding plumage. Red knots, many of them anyway, fly from the southern tip of South America to the Arctic for breeding season, then back to South America—a route that reaches 20,000 miles. It’s not the longest, but for a bird that weighs about as much as a baseball, it’s exceptional. But a flight like this requires a great deal of calories. Red knots burn nearly half their body weight long before they reach their destination. To make it to their Arctic breeding grounds in good health, let alone at all, they need a significant source of food along their route. For this, they turn to horseshoe crabs.

On the Atlantic coast of the United States, particularly in the Delaware Bay, thousands of the uncomely sea creatures come ashore on a late spring full moon to lay their eggs. This abundance of eggs is the perfect fuel for the red knot’s final jaunt to the Arctic.

The first thing every red knot does is hatch. This happens one summer in a nest that’s been scratched into the ground somewhere in the Arctic. Nests contain three or four eggs but even in the best of times, only half of these might hatch. Then among those that do, a few will become food for something else—foxes, maybe bigger, badder birds.

But that’s why they breed up here in the first place. Not to become food for something else, but because they’re actually less likely to. Fewer predators live here, at least fewer predators per bird. In fact, the farther north the knots go, the higher the survival rate for the chicks. And if it’s a good lemming year—if the lemmings are plentiful, fat and especially stupid—it’s even better. Obviously not for the lemmings, but for the knots whose predators are off chasing these larger serving sizes.

Above all, there’s more food for the knots themselves. The Arctic actually has quite a bit of food, at least for a little bit and especially if you’re a bird. Summer and its long daylight hours bring spiders, flies, swarms of mosquitoes, and other insects. Plus with all the daylight, there’s more time to hunt for food. If you’re a knot looking to raise some little knots and feed your growing family, the Arctic is as good as it gets. So as summer arrives in the Arctic, so do flocks and flocks of knots.

Anyway, the little knots hatch and they’re off and running then. Knot chicks leave the nest just hours after hatching and within a couple of weeks, they’re flying. When the chicks are a few days old, mother knot heads south. The males stay a bit longer to keeping the chicks warm and safe from predators, but within a few weeks, they’re gone too. Now it’s just the young. Soon they’ll take off on their own, even though they’re only several weeks old, and in one of nature’s many wonderful mysteries, they’ll navigate their way thousands of miles south on their own for the first time.

For many knots the first layover on the flight south is the Mingan Archipelago, a string of roughly 40 islands in eastern Canada at the mouth of the St. Lawrence River. Naturally, Mingan’s attraction is food. For southbound knots that need to bulk up, Mingan’s buffet of shrimp, snails, clams and mussels is invaluable.

Mingan isn’t the only stopover for red knots though. In fact, many won’t stop here at all, but instead find other food-rich locations down the Atlantic coast of the Americas. Red knots don’t all go to the same place either. Some will stop partway down South America, some only go to the Caribbean, and some—the most lethargic, apathetic knots—stop as far north as Florida. But for many knots, more than half of them, South America’s southern tip is the ultimate destination. The shores of Patagonia and especially the remote Tierra del Fuego are a rich source of food. The birds spend their days finding snails, worms, clams, and crabs, but the ultimate attraction is young mussels (called spat) whose shells aren’t fully developed and are easy to digest. The spat cling to the ocean floor and when the tide goes out, the knots get to work.

There’s another advantage for birds this far south, the same one that draws them to Arctic summers—daylight. This far south, many days have two well-lit low tides, so again, there’s a great deal of food and plenty of time to find it.

Summers in the southern hemisphere are spent hunting in the rhythm of the tides, resting, and fattening up. As the southern summer comes to an end, the knots gorge themselves, bulking up for their monumental flight north. As the knots get pudgy, they also grow restless. By March, summer’s over in the southern hemisphere, but summer’s on it’s way in the Arctic. Soon the knots will be too.

As red knots begin their flight north, horseshoe crabs are making their way ashore for spawning season. Horseshoe crabs spend their time on the ocean floor off the east coast of the United States, Mexico, and Central America, but during spawning season they congregate along the shoreline. Durig this time, the largest concentration by far is on the shores of the Delaware Bay. The peak of spawning season here is mid-May to mid-June at the highest of high tides—full and new moons. Spawning at peak high tides gives the eggs time to hatch before being washed away.

The spawning process isn’t sophisticated. Males dawdle in shallow water waiting for a female to crawl ashore. When one does, the male latches himself to her and and hitches a ride. The female crab lays thousands of little green eggs in the sand, positions the male to fertilize them, then covers them up with sand. As the tide recedes, she repeats the process.

The eggs are laid several inches under the surface, but the surf and other spawning action resurface many. A female might lay as many as fifteen batches of eggs in a spawning season, some 80,000 in all, but nearly all of these will die. On average one will hatch and survive a full year. It’s really just a numbers game.

Somehow or other, red knots know where to be and when to be there. They arrive at Delaware Bay haggard—shrunken gizzards, underweight, sometimes emaciated. What they desperately need is energy rich, easily-digestible food, and as it happens, by the peak of spawning season, millions of horseshoe crabs have laid billions of soft, fatty eggs. Essentially, the beach is covered in bird food.

After feasting on horseshoe crab eggs, the knots will fly the last leg of their journey to the Arctic, raise their young, and do the whole confounded thing again. Knot migrations are herculean. To make it to the Arctic, the tiny birds fly nearly 10,000 miles. To make matters worse, they must work out the navigation and logistics of it all. They might need to find their way back on track after being blown hundreds of miles off course by violent storms, or travel thousands of nonstop miles spending several days at a time aloft. Their world is one of feast and famine, especially during migrations, and horseshoe crabs are a vital link in their food chain.

This important food source has been under stress from one thing or another for years though. Really, it’s just been one thing—people—but the reasons are varied. Since the 1800s, horseshoe crabs have been caught and pulverized for fertilizer and chicken feed, used as a favorite bait to catch eel and whelk and most recently, donated blood involuntarily for LAL—an important element for medical sterility tests. Horseshoe crab populations have fluctuated over the years, and red knot populations reflected this, but the birds have been resilient.

For a pint-sized shorebird that flies more than 15,000 miles year after year, what else would you expect?

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