The epic journeys of the plover
The plover's extraordinary endurance flights can cover nearly 30,000 kilometres. Eric Wagner investigates how it can achieve such feats and how long it can keep on doing so.
On 19 June 2014, a few miles outside of Nome, Alaska, a biologist named Wally Johnson trapped a male Pacific Golden Plover sitting on a nest. After weighing and measuring the bird, Johnson attached a small satellite transmitter to his back and let him go.
The transmitter’s identification number ended in 97, so Johnson called the plover Number 97. Number 97 stayed in Nome for another month, helping his mate incubate their clutch and tend their chicks. In early September he left Nome and flew south to the Yukon-Kuskokwim Delta. There he joined thousands of other shorebirds as they prepared to migrate to places throughout North and South America, Asia and Oceania.
On 15 September, Number 97 left and flew towards Hawaii, following a direct route until he was above the archipelago, when he veered west, perhaps to skirt a storm. He traced a shallow arc across the Pacific Ocean, covering more than 8,800 kilometres before he landed in Okinawa, Japan, on 23 September. It was the first time he had touched land in eight days.
Number 97 stayed in Okinawa for the next 26 days before leaving for Sulawesi, an island in Indonesia. He arrived on 28 October, established his territory on a rice field, and set about eating insects and worms.
By March, Number 97 was restless. He left on his northward migration on 3 April. He stopped near Hong Kong, then spent a few days near Shanghai. He next flew east over the Yellow Sea and the Korean Peninsula, then ducked into northeastern China, staying for 10 days. Finally, he left for Nome, touching down at 9.00 pm on 31 May. He had flown more than 27,000 kilometres.
Johnson has studied these birds for nearly 40 years, but this flight astonished him. “I’d never seen a path like that,” he says. “It was amazing – more so because other plovers probably do it, too.” Only within the past few years have scientists begun to grasp the incredible journeys of which shorebirds are capable. Their non-stop flights take them farther and faster than any other creature, even rivalling the greatest human feats of aerial endurance. Plover migrations recall the 1986 flight of Rutan Model 76 Voyager, the first aircraft to fly around the world without refueling. The aircraft’s nine-day, 42,200-kilometre flight captured the public’s imagination. Shorebirds are supremely optimised for such acts, testing the definitions of extreme behaviour.
But even as Johnson revels in the natural history he is helping to describe, he is keenly aware that it is in danger of disappearing. Half of all shorebird populations worldwide are declining. The threats are familiar: climate change, which will likely be detrimental to tundra vegetation and its insect life and may also upend the winds that shorebirds depend on, and habitat loss, which deprives them of their refueling stops. Johnson finds himself asking two questions: How do these birds achieve their astonishing flights? And how much longer will they be able to continue?
As Johnson and his colleagues race to find answers, migrating shorebirds are forcing us to broaden our understanding of the possible.
Their non-stop flights take them further
and faster than any other creature.
Pacific Golden Plovers are slightly larger than common blackbirds. They breed throughout much of Siberia and along Alaska’s western coast. When adult males are in breeding plumage, as they are during the brief Nome summer, they become, I think, the most striking birds on the tundra. Their backs are flecked with gold. Their faces, chests and bellies are as black as oil. A band of white that has been likened to a barrister’s wig runs from the plover’s brow and down its flanks. (Females have the same colour scheme, but are paler.)
Johnson, who is now 80, has studied plovers since the 1970s when he was a professor at Moorhead State University in Minnesota. He saw his first plover while working in the Marshall Islands in Micronesia. The bird captivated him. Here, before him, was a species that nobody knew much about. Johnson is a physiologist and histologist – someone who studies the body at the organ-wide and intracellular level. The plover’s internal machinery – the way it uses body fat to fuel its migration, for instance – was what interested Johnson at first. But his attention moved from the plover’s physiological engine to its wayfaring lifestyle. Johnson retired from teaching 20 years ago, but has not stopped following plover migrations. Asked why he hasn’t stopped, Johnson confesses: “I can’t.”
The plover’s life cycle involves vast geographic shifts. They breed in the far north, disappear for a while, then reappear thousands of kilometres away. Johnson wondered: Are their flights non-stop? What paths do they follow?
The notion that plovers and other shorebirds might migrate great distances without stopping had tantalised biologists for decades. All kinds of animals migrate –birds, mammals, fish, butterflies. They venture far afield to find food, or more favourable climes, or to breed. But shorebirds take the practice beyond even its logical extreme. How their behaviour evolved is a tricky question. Did it happen gradually, with birds that already migrated long distances overland slowly expanding their breeding ranges across the seas, say from Siberia to Alaska, obligating them to fly across the oceans as well? Or has the migratory flight itself slowly lengthened, as birds flew from their breeding grounds to new non-breeding sites that were farther and farther south and east from the Arctic? No one can say for sure, although biologists do know the behaviour has arisen independently several times, and in several taxa of shorebirds.
In 1989 Johnson calculated that the plover’s maximum non-stop flight range might be about 9,200 kilometres. Other researchers, too, suggested that shorebirds might journey great distances in a single flight. The evidence, while circumstantial, was powerful and connected with the remarkable physiological transformation shorebirds experience before they leave. The birds gorge themselves when they gather at their staging areas; golden plovers can more than double their weight before migratory flights. The food the birds seek provides fuel as well as other, secondary benefits. Some of the amphipods on which semipalmated sandpipers feast, for example, contain fatty acids that may increase the birds’ aerobic capacity. (Anders Hedenström, a Swedish biologist, has likened this to “self-doping”.) By the time the birds leave, more than half their mass might be fat, with yellowish deposits straining against the skin. Like the aircraft Rutan Voyager, they jettison all excess equipment. The organs that are not needed for their flight – the gizzard, the digestive system and the like – atrophy to shave precious grams. As Hedenström has written, migrating shorebirds are “quite close to the ‘optimal design’” for long-distance, non-stop flights.
Still, definitive proof of extended, non-stop flight was elusive. Investigations into the achievements of plovers has become possible in recent years as, in Johnson’s words, the equipment used by researchers “has gotten smaller and lighter”. The technologies in question are minuscule geolocators and satellite transmitters, which have shrunk so much that birds weighing only a few hundred grams can carry them without being unduly burdened. The instruments’ influence has been profound. In 2007, biologists surgically implanted a satellite transmitter in a female bar-tailed godwit. The godwit in question was a bit smaller than a crow. It flew nearly 12,000 kilometres from Alaska to New Zealand in close to nine days – the longest recorded non-stop flight made by any bird.
Subsequent work has shown the remarkable godwit has long-distance rivals. Johnson found that some plovers probably fly non-stop from Alaska all the way to Queensland, Australia – a trip of some 11,000 kilometres. Much smaller sharp-tailed sandpipers may make a similarly long non-stop flight, as juveniles no less. A squat shorebird called a ruddy turnstone flies from the Arctic to Australia. What once struck scientists as incredible is, it seems, routine for several species.
What once struck scientists as incredible is, it seems,
routine for several species.
“If you look at almost any shorebird in the Pacific basin, you’re going to find a migration strategy that falls into the extreme endurance side of things,” says Bob Gill, a biologist with the US Geological Survey (USGS) who led the team that uncovered the godwit’s flight. And this has led to another question: how might these birds find their way across the vast, seemingly featureless Pacific ocean? To find an answer, some biologists are turning to another species: homing pigeons.
On 29 June 1997, Britain’s Royal Pigeon Racing Association held a race to celebrate its centenary. At 6.30 am, more than 60,000 pigeons were released in France to fly up to 800 kilometres back to their lofts in England. By late morning they were heading over the English Channel. The quickest pigeons were expected to arrive at their lofts by early afternoon.
Over the next few days, a few thousand birds trickled back, but tens of thousands were never seen again. In the parlance of a pigeon race, this is called a smash, and the 1997 royal pigeon smash was one of the worst ever recorded. When Jonathan Hagstrum, a USGS geophysicist with a side interest in bird navigation, read of the smash a year later,
he had a hunch about its cause. “I was looking for an acoustic source,” he says. When he checked airline schedules, his hunch was confirmed: on that June day, a Concorde jet had taken off from Paris and flown across the path of the racing pigeons. The sonic boom, Hagstrum says, disrupted what he calls the pigeons’ infrasonic map – one that relies on low-frequency soundwaves. His finding was published in the Journal of Experimental Biology in 2000.
Any creature needs a compass and a map to navigate. Birds use several compasses: they can find their way by the night stars, or the Sun’s position during the day. They can also sense the Earth’s magnetic field, which offers them a built-in sense of direction. But, as Hagstrum points out, a compass only points north or south. It cannot tell you where you want to go. “For that, you need a map,” he says. “So the big mystery is: what is the map?”
Scientists have proposed two potential maps. Birds detect consistent variations in the Earth’s magnetic field and may use them to generate a kind of X-Y coordinate system. They may also plot their flyways by piecing together olfactory cues – a whiff of a cypress forest, a city socked in smog.
The problem is the proposed maps don’t seem applicable to shorebirds. In the Pacific basin the magnetic map is – for complex geophysical reasons – unavailable. And although scent maps might work well on smaller scales, the idea that such a map could guide a bird from Alaska to Australia, across a vast ocean and through unstable atmosphere strains credulity, Hagstrum says.
He thinks he has found a new map that may help account for shorebirds’ remarkable long-distance sojourns: the Earth’s low-frequency background noise, known as infrasound. Infrasounds include the heavy murmur of the planet’s unceasing seismic activity, or the rumble of ocean waves. Such sounds can reverberate thousands of kilometres, over land or through the atmosphere. “They’re very low frequency, but ubiquitous,” Hagstrum says.
Infrasounds may be well below the range of human hearing, but birds can hear them. Hagstrum believes infrasounds generate an aural map for birds – so that the sea becomes, for instance, a vast infrasonic soundscape. It is a structured space, he argues, with predictable patterns of trade winds, equatorial winds and so on all acting as acoustic beacons. In this way, these birds can figure out where they are and where they are going. He and Gill are testing the idea, using data from migrating bar-tailed godwits to see how their routes correlate with acoustics data from the US Navy.
One reason Hagstrum’s ideas are controversial with some biologists is because they are based on correlations, rather than experiments. Shorebird biologists, however, are more receptive.
In part that’s because shorebirds are avid students of the wind. “They pay attention to atmospheric conditions and start when they have a good tailwind,” Johnson says. But their reading of the wind seems to somehow extend beyond local conditions: remarkably, Gill has found that godwits wait to leave Alaska until winds are mostly favourable along the entire length of their hemispheric migratory route. They lift off from the beach, fly around until they have sensed whatever it is they need to sense and then decide whether to return to land or continue.
Hearing this, I picture a flock of birds circling the beach, in effect sounding out a potential route that will take them over Hawaii, over the equator, and perhaps all the way to New Zealand or Australia. I think of how I have felt when I take a transoceanic flight, the sea far below, blank and forbidding. Then I think of that moment when the birds commit themselves to their journey. They don’t throw caution to the wind, but trust their fate to it. The wind is at their backs. How must those deepest of thrums sound, over a great distance?
Hagstrum thrills a bit when asked to imagine it. “I think birds might be listening to the landscape or the seascape the way we look at it, getting information from it, using it to make decisions,”he says. “For them, it’s like the song of the sea.”
The June sky is spitting rain when Johnson pulls off the road next to Anvil Mountain, near Nome. We are looking for Number 97. According to the transmitter’s daily update, Number 97 was on this hillside a few hours ago, but Johnson fears he may have gone. If that’s the case, it bodes ill for the bird. “He was late getting back to Nome, and I think he may have lost his territory to another male,” Johnson says. “He might just be wandering around.”
We spread out to search. I observe Johnson’s method as he starts up the mountain. He is tall and soft-spoken. His pace is deliberate. I try to match it. The vegetation of the tundra is wet and soft; it swallows my boots and slows my pace, allowing me to better appreciate the subtleties of the landscape. What I took to be flat, drab ground has gentle rolls and grassy tussocks, bursting with greens, yellows and pinks. Somewhere, Number 97 might lie flattened over a nest, invisible to the predatory (or scientific) eye. But after a while Johnson decides he was correct: Number 97 is not here.
We drive to another field, where Johnson thinks plovers have established territories as large as 50 hectares. This year he plans to put geolocators on five birds, filling in more details of their preferred habitats during migration. “These things are amazing,” he says, holding up a coin-size geolocator. “We’ve found out things about plovers’ routes and stopovers that are critical to their conservation.” Chief among those stopovers, he has learned, are vital refuelling sites among Japanese rice fields, where plovers bulk up on their way back to Nome.
We start surveying, and have walked for 15 minutes when I hear a high, thin whistle: Eeeeeeeep! A black head pops up 60 metres away. Johnson freezes. “He’s letting us know he sees us,” he says, and quickly sits down. The plover is not Number 97 but another bird alarmed at the intruders on his territory. He watches Johnson with an affectless animal curiosity. Now a peculiar dance begins between bird and biologist. To catch the plover we need to find his nest. We have an idea of where it is, but if we stomped over we would never find it, so well hidden are the eggs.
We wait for the plover to show us where to go. He stops eeeeeping, runs a few metres and pauses, runs a few metres, pauses again. He does this a few times before stopping with a special finality: he is at his nest. “OK, now you walk in that direction,” Johnson whispers. “When you get close he’ll get up again, so you really try to focus on the spot, but stop a few feet before you get there – you don’t want to crush the eggs.”
No, I don’t. I start walking. The plover pops up, eeeeeeping. I fix on a rock near him and stride forward. The plover flails before me, eeeeeeping urgently, pretending to have a broken wing. But I am not so easily fooled. I reach the rock and stare at the tundra’s visual chaos until I see a small cup of lichen and moss: the nest. The eggs, olive with dark green speckles, are arranged about the centre, pointed inward. Their camouflage is extraordinary.
Johnson marks the nest with a flag. We will return to set a trap for the plover and put the geolocator on him, but before we go we want to see the plover go back to his clutch. This will serve as a proximal gesture of forgiveness, a way of letting us know he is OK. We wait as he scuttles to his nest and melts into the tundra’s mosaic.
As we drive away, Johnson muses about what the future holds for these creatures, so small and delicate, but also capable of such feats of endurance. “These birds are tough, but they’re also so finely calibrated,” he says. “They are already living at the extreme edge of what they can do, and it’s a shame when we make things harder for them.”
Plovers face two main challenges: the future of the winds that carry them and changes to the places the winds carry them. Biologists worry that climate change will upset the winds shorebirds depend on to sling them across the oceans: the trade winds, the equatorial currents, the lower bounds of the jet stream and so on, all of them forming a kind of conveyor belt across the hemispheres. How those effects will manifest is unknown, but climate models project shifts of varying severity, as some winds slacken and others increase. For creatures so attuned to the weather, even small shifts matter.
On top of large-scale changes, shorebirds may also contend with sudden, less predictable weather events. Most models predict that as the climate changes storms will spring up more frequently over the oceans, as one did during Number 97’s journey. Such storms could blow birds off course to unfamiliar lands that might not suit their needs so well. Although Number 97 wasn’t deposited someplace inhospitable, Johnson suspects the detour knocked him off schedule, which may explain how he lost his territory and is now so hard to find. “Birds have been making these flights on these oceanic flyways for a long time,” Johnson says. True, many species are long-lived – the oldest known golden plover was 21 years old – and they are accustomed to day-to-day changes in the weather. But while they may be adept at withstanding short-term anomalies in wind speed and direction, it’s an open question whether they are flexible enough to keep up with future meteorological shifts.
“They are already living at the extreme edge of what they can do.”
An even more pressing issue is habitat loss. The effects of climate change on shorebirds are – for the time being – largely theoretical, but wetlands are disappearing now. In the Pacific basin, many shorebirds fly to their northern breeding grounds via the East Asian coast. Their northward flights are not as spectacular as their southward ones, but they are still long, requiring refuelling stops as the birds prepare to breed. One of the most important stopovers on this East Asian-Australasian flyway is the complex of estuaries and tidal flats on the Yellow Sea, between China and the Korean Peninsula. More than 40% of individuals from some species depend on a single 180,000-hectare coastal wetland at the mouth of the Yalu River.
“For many shorebirds, that is the only place they stop when going north,” says Gill, the USGS biologist. The food they pick out of the wetland mud “is what gets them to the breeding grounds”. Unfortunately for the shorebirds, these vital checkpoints in China and South Korea are also subject to intense development pressures. Many of the wetlands are being filled in with sand, walled off from the tides, drained and built upon. In spite of a series of international agreements designed to slow development, 65% of coastal wetlands have disappeared in the past 50 years and the rate of loss is accelerating. Also, rising sea levels may inundate up to 40% of suitable mudflats and marshes. Deprived of their former stopping sites, tired and hungry shorebirds must find other places to feed and rest as they head north. Whether they will succeed is, again, unclear.
Two days later, we are back to capture the plover and place the geolocator on him. We find his nest again; the male stands near us, eeeeeeeping. Paul Brusseau, Johnson’s former student, sets a trap. The trap is a metal ring of Russian design called a “luchok”, the Russian word for “hoop”. Brusseau first replaces the plover’s eggs with wooden dummies and arranges the hoop over the nest, with fishing line across the dummy eggs. When the plover settles on the eggs, the trap will spring and a net will snare him.
We retreat and wait as the plover goes through his coy routine of return. When he sits on the eggs at last the trap springs and Brusseau rushes over to extract him. In the van, Johnson weighs him and measures the length of his wing and leg. Although it is still early in the breeding season, the bird is thin, his feathers ragged. Johnson puts a series of coloured bands and a geolocator on his slender legs. The geolocator will track the plover when he leaves Alaska for the Central or South Pacific and follow him as he comes back to this same field next year.
The transmitter’s frequency ends in 36, and so I think of the plover as Number 36. “You want to let him go?” Johnson asks, holding out Number 36.
Of course I do. I clasp Number 36 in my hands, pinning his wings to his sides. I can feel his sternum, and the panicked tremor of his heart. He is impossibly light; Johnson found that he weighed 116 grams. Before he leaves he will fatten to perhaps 200 grams, but he seems too fragile now. The thought of Number 36 winging across the Pacific gives me pause, and I am reluctant to let him go. But this is a test of my understanding and imagination, not his. He has made the trip before. He will do it again, maybe 10, 15, 20 more times. He struggles against my grip. I open my hands and he flings himself away.
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