A tight, fast-flying group of 15 small, gray birds appears out of the sky over the vast coastal mudflats of Mauritania’s Banc d’Arguin National Park, where the western edge of the Sahara meets the Atlantic Ocean. They circle around together, extend their long, thin legs, and flutter down to land; these young red knots have just concluded an epic trek that began in northern Siberia and passed through Europe.
The birds consumed more than an ounce of their five-and-a-half-ounce body mass to power their ever-pumping flight muscles over the course of the journey, and now, having lost so much weight, they need to eat. They quickly tidy their feathers and begin probing their long, thin shorebird bills into the wet mud.
They also begin to remake their bodies: They start breaking down the large pectoral muscles that they’d developed for their long flight and reallocating proteins to build an exceptionally strong gizzard, a stomach-like chamber in their digestive system. They need a powerful gizzard to crush and digest nutritious Loripes clams, which live below the surface of the mudflats and are the most abundant source of high-quality nutrition on the knots’ wintering grounds.
Only 100,000 wintering red knots were counted in the Mauritanian mudflats in 2022, compared to the half million counted in the 1980s.
Juvenile knots stay on the Banc D’Arguin for about 20 months as they mature. They rely on other, less nutritious foods to sustain them while their bodies change and they become more practiced at finding and digesting clams. By the time they reach adulthood, Loripes should make up the majority of their diet, and the knots should be thriving, strong, and ready to fly north to breed for the first time.
But the red knots on the Banc d’Arguin mudflats are not thriving. Only a hundred thousand were counted here in 2022 compared to the half million or more that regularly wintered here in the 1980s. Loripes clams still populate the tidal flats, young knots still build strong gizzards, but nowadays the birds feed heavily on low-protein Dwarf Eelgrass, a plant that also grows in the mud. This isn’t a matter of choice for the birds: They can’t easily feed on the clams anymore because in northern Siberia, well over 5,000 miles away, the climate is changing fast.
The Afro-Siberian red knot (Calidris canutus canutus), one of six red knot subspecies, migrates annually to the Taimyr Peninsula in far northern Siberia to breed. For thousands of years its members timed their northward flights to arrive at this massive hammerhead of low-lying land on the edge of the Arctic Ocean just as the winter snow begins to melt and spring begins to tease the tundra vegetation — and the vast number of small invertebrates that live in it — into life again.
Banc d'Arguin National Park in Mauritania, wintering grounds for Afro-Siberian red knots. Hemis / Alamy Stock Photo
As with many migrant species that breed in strongly seasonal climates, Afro-Siberian red knot reproduction is a speedy, tightly scheduled affair. As soon as the birds arrive at their breeding grounds, the males occupy territories and start attracting females. Within a week the birds have paired up, made nests — just small scrapes on the ground — and begun laying a clutch of (usually) four eggs at the rate of one egg per day. Males and females alternate egg-incubation duties, and their young hatch three weeks after the clutch is complete, just when tundra insect populations have, for many years, reached their spring peak.
As soon as the young emerge from their eggs, mother knots leave them and begin flying south. Males are left alone to raise the babies, which start out as tiny balls of fluff, perfectly camouflaged in the low tundra vegetation. Although the hatchlings are precocial — able to walk around and find their own food almost as soon as they break out of their eggs — their tiny size means they have little thermal inertia. They lose energy very quickly to the surrounding cold air, and they’re unable to generate enough metabolic thermal energy inside their bodies to make up for the losses. So for their first 10 days of life, their father must periodically gather them together under his wings and warm them up. He also keeps a lookout and warns them of approaching predators, like Arctic Foxes, uttering a sharp peep that sends them down into the tundra, motionless and nearly invisible, until danger passes.
Young knots feed voraciously on tundra insects. They reach full size and are able to fly in just three weeks, at which point their fathers fly south, leaving them alone to bulk up on the last remaining invertebrates of the short Arctic summer. A couple of weeks later, with the snow and ice closing in, the young birds too start flying south, their genetically coded instincts routing them via the vast Gdansk wetlands on the Polish coast and the Dutch Wadden Sea to Mauritania in northwest Africa.
Juvenile knots arriving in Mauritania in 2015 were a remarkable 20 percent smaller and lighter than those arriving three decades earlier.
This schedule worked well for Afro-Siberian red knots for a very long time. But in recent decades the Arctic has been warming three to four times faster than the average for the rest of the globe. Spring snowmelt has been arriving progressively earlier on the Taimyr Peninsula; scientists found that between 1983 and 2015, it advanced by half a day per year on average. Spring warming also progresses erratically, sometimes interrupted by intense cold snaps that kill many insects. So the spring insect peak not only comes weeks before it used to but also produces insects far less reliably than before.
Fewer insects means that adult male red knots have less energy to keep their young warm and safe in their vital early weeks. Many fathers abandon their young prematurely and fly south to more stable feeding grounds. Young knots are now more likely to be malnourished, and many perish before they learn to fly. Ornithologists found that juvenile knots that arrived in Mauritania in 2015 were a remarkable 20 percent smaller and lighter than those arriving in the early 1980s. Their average beak length also shrank, though by only 10 percent; they were about three millimeters, about an eighth of an inch, shorter than before.
A male Afro-Siberian red knot sits on a nest with chicks in the Arctic. Jan van de Kam
Bill length theoretically means little on the tundra — shorter-beaked knots should catch insects just as well as longer-beaked knots — but it becomes critical once they arrive in Mauritania. It turns out that knots with shorter bills can’t probe quite deep enough into the mud to reach the majority of Loripes clams. Biologists have found that knots with 30-millimeter-long beaks can reach only about a third of the Loripes in the mudflats, whereas birds with 40-millimeter-long beaks can reach more than two-thirds of the available clams. Instead of shifting to this high-protein food, an increasing number of young red knots is forced to continue eating a lot of Dwarf Eelgrass — which is found on the surface — and smaller, less abundant types of shellfish during their time on the Banc D’Arguin. They simply don’t get enough nutrition, especially protein, and over months they start wasting away. Many die.
Although its average body size is decreasing, the Afro-Siberian red knot is under sustained evolutionary pressure to keep its bill as long as possible. Birds that can reach more Loripes clams survive and breed far better than their shorter-billed species-mates, which is why bill length is decreasing more slowly than body weight.
Climate breakdown is affecting migratory species all over the world. Many species of long-distance migrant birds are evolving smaller bodies and longer wings, to move farther, faster, on less food. Many species are also sliding out of sync with their seasonal food sources and suffering reduced breeding performance, declining health, or reduced longevity. Others are successfully changing their migration routes and timing, and yet others are no longer migrating at all, usually because warming winters are making long, dangerous journeys unnecessary.
If red knots arrive at their traditional breeding grounds in Siberia just a few days too early, they will starve or freeze to death.
Red knots are changing with the climate, at least somewhat. They’ve been arriving on their Taimyr Peninsula breeding grounds earlier than they did in the 1980s, as the climate warms. But they’re not making up enough time. Their rate of arrival advance is only half of the rate of snowmelt advance, an average of a quarter of a day per year versus half a day per year. They’ve tried to compensate for this by nesting at progressively higher altitudes than before, selecting cooler sites where insect emergences are slightly delayed (each 10 meters of altitude delays insect emergence by about a day). But the Taimyr’s tallest hills top out at only 50 meters above sea level, and the knots have already started nesting there. They have nowhere higher or cooler to go.
Why haven’t the knots kept up with the ever-earlier snowmelt? Scientists believe it may be because of the extremely high cost of arriving in the Arctic just a little too early. There is absolutely nothing for the birds to eat when snow and ice still cover the ground; insects are abundant only after snowmelt. If red knots arrive at their traditional breeding grounds just a few days too early, they will not merely suffer inconvenience or reduced breeding performance — they will starve or freeze to death.
The red knot as a species had to have been somewhat schedule-flexible to survive earlier changes in climate, but knots with just a little too much flexibility in their migration schedules would have rapidly been eliminated from the gene pool. It stands to reason that the red knot population could over time have become dominated by individuals carrying genes that constrained the extent to which they could deviate from established migration schedules; this would function as a governor, limiting the rate at which the population’s schedule could change.
A flock of red knots at Snettisham nature reserve in England. Clive Dodd / Alamy Stock Photo
But current, human-caused Arctic warming is proceeding much faster than any natural warming episode that red knots have experienced in their species’ entire existence. It’s likely that the mechanisms that successfully moderated their migratory evolution in the past are now preventing them from changing fast enough to survive.
A few decades ago, when the Afro-Siberian red knot population was stable and its Arctic breeding area wasn’t yet noticeably warming, the subspecies would normally lose about 15 percent of its adult population every year to predation, migration accidents, and natural death. Thus, to maintain its numbers, juveniles equivalent to at least 15 percent of the population would have to survive to adulthood every year. Observers would therefore expect that at least 15 percent of a healthy population would be juveniles.
Researchers are finding fewer male than female red knot chicks hatching in Siberia, though the reasons for this are not fully understood.
Scientists censused the knots wintering on the Banc d’Arguin in early 2022, as they have done annually since the early 1980s. Besides confirming an ongoing decline in numbers, they found that less than 1 percent of the birds were juveniles. They also observed a rapidly growing sex imbalance: In 2022 there were almost three females for every male even though there had been equal numbers of both sexes just 20 years earlier.
This sex imbalance may be at least partially explained by the fact that males are slightly smaller and have slightly shorter bills than females, so they’re able to reach even fewer Loripes clams and are likely even less well-nourished and able to survive than females. But researchers are also finding fewer male than female chicks hatching in Siberia, though the reasons for this are not fully understood.
Because red knots are monogamous, mateless females are unable to breed. They will fly the dangerous course between the Arctic and Africa again and again, and produce no young. We are watching as the Afro-Siberian red knot, this branch of a species that has survived tens of thousands of years of climatic ups and downs, heads into extinction within our lifetimes.
Adapted from The End of Eden: Wild Nature in the Age of Climate Breakdown, by Adam Welz, published by Bloomsbury Publishing. Copyright © 2023 by Adam Welz.
