Afro-Siberian red knots migrate from the Arctic to winter in Africa, where they recover from the arduous journey. But warming in Siberia is causing physical changes in the birds that hinder their ability to feed, and scientists fear the subspecies is headed for extinction.
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.
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.
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.
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.
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% smaller and lighter than those arriving in the early 1980s. Their average beak length also shrank, though by only 10%; they were about three millimeters, about an eighth of an inch, shorter than before.
Bill length 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.
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.
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% 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.
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.