Bog walking is a treacherous business. The bog, or muskeg, near Beluga, Alaska, is a floating mass of vegetation, grassy hummocks and stunted black spruce trees that stretch for miles in every direction, with the snow-flecked mountains of the Alaska Range shining in the distance. Few trails exist. Walking here is like sloshing across a very wet sponge, as each step sinks into a few inches of water. It feels as if the ground might give way. Sometimes it does. A wrong step can sink the uninitiated into thigh-deep water that requires a hand up. Clouds of mosquitoes search for any bit of exposed flesh. Rangy moose emerge from groves of dwarf trees to threaten trespassers.
This, however, is where one of the world’s premier ultra-endurance athletes lives.
Long-distance migration is the most extreme and life-threatening thing that any animal does. And migratory shorebirds make the most miraculous journeys of all, given the distances they cover and their tiny size. There are some 70 species of shorebirds in the world that make the journey from the top of the globe to the bottom and back every year.
The Hudsonian godwit is one of them. Named after the Canadian bay where the species was first identified, and the bird’s distinctive two-syllable cry (“god-wiiit!”), Hudsonian godwits lay their eggs each spring in this Alaskan bog. (All godwits breed in the Northern Hemisphere.) In June or July, they leave their self-sufficient hatchlings and head south. First, they fly for three days to the wetlands of Saskatchewan and feed for one month. Then they continue down through the Americas to the northern Amazon—a 4,000-mile trip. They feed again and a week later head to Argentina, feeding another time before continuing over the Andes to Chiloé Island, on the fecund Gulf of Ancud, where they arrive in September or October and winter for a little over six months.
The longest leg of their journey, some 6,000 miles, is on the return from Chile. They fly night and day at speeds between 29 and 50 miles per hour, not stopping to eat, drink or rest. They pause for a couple of weeks to refuel in wetlands in the central United States—usually Nebraska, South Dakota, Kansas or Oklahoma—and then continue back to the Alaskan bog. Their goal is an endless summer.
In person, the Hudsonian godwit is prepossessing, sleek and reddish brown and gold in its Alaskan spring breeding colors, with slender stiltlike legs and a very long, upturned bill specially designed for feeding in mud. If you are a researcher trying to capture and study baby Hudsonian godwits, you’ll look for a well-camouflaged, soup-cup-size nest on the ground. Once you find it, you’ll get close enough to the mother bird to scare her into flight.
That’s why Nathan Senner, an assistant professor of ornithology at the University of South Carolina, is patiently and doggedly slogging through this swamp, clad in army-green hip boots. He’s accompanied by his wife and fellow ornithologist, Maria Stager, and master’s student Lauren Puleo. They are waiting for the elegant, long-legged, long-billed mother bird to fly up, shrieking and scold-ing, leaving its four—almost always four—moss-brown eggs exposed.
A problem with this tactic is that some females are so hard-wired to protect their brood, they won’t fly away from the nest entirely, even when you’re close enough to step on them.
“So the holy grail for finding a nest is the incubation switch,” Senner explains. That happens when the male returns home from a day feeding on mollusks and marine worms on the vast shimmering mud flats in the nearby Gulf of Alaska. The researchers spot him as he zooms in to take over nest duties so the female can feed. “You see the male go to the ground and the female pop up. And, aha, there’s the nest.”
Tromping through the bog last spring, Senner and his team found 15 nests. No small feat: It takes 24 person hours of searching in the perpetual twilight of late spring in Alaska to find just one. When a nest is found, Senner picks up and floats each egg in a small clear plastic cup filled with water. In one case, an angry mother godwit casts an agitated eye from the top of a nearby stunted tree and dive bombs the research crew to defend her nest. Senner nonchalantly ducks the angry bird and continues his work. The height and angle of the floating egg indicates when the chicks will hatch: His goal is to come back and find the babies within hours after they’ve pecked their way out of their shells.
Robins, bluebirds, hummingbirds and many other birds are altricial. That means they are helpless at birth, featherless, eyes closed and nest-bound, and they need parental care before they learn to fly. Godwits and most other ground nesters, on the other hand, are precocial birds. The hatchlings are raring to go a couple of hours after they emerge, running across the bog snapping at swarms of mosquitoes and flies. This immediate self-sufficiency also helps them avoid being gobbled up in the nest by a voracious mew gull or a fox. Either way, after those first few hours, they’ll be hard to find and therefore lost to the cause of research. Senner isn’t worried. “I have literally never missed a hatch,” he says.
A newly hatched Hudsonian godwit weighs less than one ounce, though before it sets off on its journey to the other end of the world, it bulks itself up to more than 12 times that weight. Scientists who study these birds readily confess to not knowing many important facts about them—from where some of them stop over to how they use magnetic forces, how they read weather systems and, in general, how the hell they can possibly do what they do. Answers to initial questions often spawn a flock of more vexing ones.
A big reason Senner comes here in the spring is to suss out the forces that are driving down the numbers of Hudsonian godwits and other migratory shorebird species. In 2018, he and John W. Fitzpatrick, the recently retired executive director of one of the world’s leading bird research institutions, the Cornell Lab of Ornithology, co-authored a New York Times op-ed about migratory shorebirds that concluded, “Numbers of some species are falling so quickly that many biologists fear an imminent planet-wide wave of extinctions.” This free fall—which Senner and Fitzpatrick call the “number one conservation crisis facing birds”—is why far-flung teams of researchers around the globe are striving with more urgency than ever to unravel the myriad mysteries of migratory shorebirds.
Senner has been immersed in the world of migratory shorebirds almost all of his life. “I grew up with godwits,” he says. He was a serious birdwatcher as a youngster, hiking with his parents on a coastal trail near his Anchorange home, where godwits were a common sight. His father, Stanley, was the executive director of the Audubon state office in Alaska, and was part of the scientific team that responded to the Exxon Valdez oil spill in 1989. Young Nate grew up with avian flashcards and dinner conversations about the challenge of protecting birds.
Another of Senner’s passions has further cemented his kinship with these migrants: He runs marathons, which gives him even more respect for the marathoners of the bird world. (His best time is an impressive 2:29.10.)
Senner is 40 and whip-thin, with a ready grin and a wealth of bird knowledge in his head. As an undergraduate at Carleton College, he worked on shorebird biology with the United States Geological Survey (USGS) in Alaska. He earned his PhD at Cornell and worked as a postdoc at the Global Flyway Network with famed Dutch researcher Theunis Piersma at the University of Groningen.
Because he grew up in Anchorage with Hudsonian godwits right outside his back door, Senner says he never thought too much about them until he learned that “most people thought godwits were really special, super mysterious and one of the rarest birds in North America, because they had never seen one. They came to Alaska to see them. And so what attracted me to them was this mystery that I knew something about and could help out with.” He became more curious, too, about where the godwits went when they left.
After he earned his undergraduate degree, he was awarded a Thomas J. Watson fellowship, which funds recent graduates to travel the world to follow their passions. He went to Brazil, Peru and elsewhere in South America to see what he could find out about godwit migration. The tracking technology was still in its early stages then, but before long, he says, there was “an explosion of technology that allowed us to blow the doors off all of these questions which we had no idea about.”
A breakthrough in miniature tracking technology allowed researchers to see precisely where migratory birds went on their journeys. This was no small revelation. Where birds went when they left one place and returned months later was for centuries a confounding enigma. While some cultures, such as the ancient ocean-going Polynesians, had knowledge of seasonal bird routes, most of the world was in the dark.
Some people speculated birds hibernated rather than migrated. One 16th-century Swedish bishop, Olaus Magnus, hypothesized birds sank into the mud at the bottom of lakes or rivers, rising to the surface at the onset of spring. One 17th-century scientist, Charles Morton, believed the birds made a lunar landing. “Whither should these creatures go,” he wrote, “unless it were to the moon.”
It wasn’t until the 18th century that experts began to realize that some birds were traveling to warmer climes where food was more abundant. That fact was driven home dramatically in 1822 when a hunter in Germany downed a white stork and discovered it had been impaled by an African spear—it had flown, wounded, some 2,000 miles from Africa to Europe. The Germans called it a pfeilstorch, or arrow-stork, and remarkably, 24 more pfeilstorchs have been found in Europe in the years since.
Bird banding became the first method of shedding light on the migration mystery. In the early 1800s, John James Audubon found that birds he banded with strings tied to their legs returned to their nest the following year. The first banding for scientific research is credited to the Smithsonian researcher Paul Bartsch, who in 1902 placed metal bands on the legs of 23 black-crowned night herons with a serial number and a message—“Return to Smithsonian Institution.”
In 1909, the American Bird Banding Association assumed oversight of all banding efforts, until, in 1920, the federal Bureau of Biological Survey created the Section of Distribution and Migration of Birds. A researcher there named Frederick C. Lincoln created the modern science of bird tracking, improving banding methods as well as recordkeeping, and used the data to advocate for the conservation of migratory birds.
Biologists once thought the Hudsonian godwit was one of the rarest species in North America, because most of its breeding grounds were in remote parts of Northern Canada. In the 1980s, researchers started seeing Hudsonian godwits that had been banded in Canada showing up in Argentina. Other godwits, without bands, were also showing up in Chile—as it turned out, those were the ones migrating from Alaska. It was still a mystery where they’d been in between.
In 1989, the science of bird tracking took a quantum leap when albatrosses became the first birds to be fixed with satellite tracking telemetry devices. A few years later, in 1994, USDA Forest Service biologist Brian Woodbridge placed a tracking device the weight of a silver dollar on two Swainson’s hawks in Butte Valley, California. A growing number of the birds weren’t returning from their winter migration and the biologist was concerned. He’d known for years that these insect-eating “grasshopper hawks” went somewhere on the pampas of Argentina in the winter, but not precisely where. When he and his colleagues used GPS tags to find the birds’ precise location in Argentina, they were shocked to arrive and find hundreds, and then thousands, of Swainson’s hawks dead on the ground in an agricultural area. The birds had fed on grasshoppers that were being sprayed with a potent pesticide called monocrotophos, and it was killing them, almost instantly. The knowledge gained through tracking led to an international outcry. Fortunately, the Argentine government took action and the number of Swainson’s hawks recovered.
In 2007, Senner was intrigued by a study in which a team led by his mentor, USGS researcher Robert Gill, attached satellite transmitters to 16 bar-tailed godwits in New Zealand. The birds had flown 6,300 miles from New Zealand to the Yellow Sea, to feed on a massive mud flat there. After six weeks they’d continued on, flying 4,500 miles to southwestern Alaska to breed. The big surprise, though, had come at the end of the breeding season.
Departing from the Yukon-Kuskokwim Delta in western Alaska, a bird dubbed E-7 had landed more than a week later on a beach on the north island of New Zealand—without ever stopping to rest. It was the longest known nonstop migration of a land bird: eight days in the air, covering 7,250 miles. (This bird’s distance has since been bested. The current record, set in September 2021, belongs to a bar-tailed godwit that spent more than 11 days in the air, covering 8,100 miles.)
In 2008, Senner fastened something called a geo-locator to the leg of a Hudsonian godwit in Manitoba, home to another population of Hudsonian godwits. Geo-locators are less precise than GPS transmitters, but they’re cheaper, and especially small and light. They record sunrises and sunsets as the bird flies, storing information that reveals, upon its return, where exactly it traveled.
In 2009, in a central Canadian bog near Churchill, Manitoba, Senner fetched the tiny tracking device that the Hudsonian godwit had been wearing. As he took the device off the bird’s leg, he wondered excitedly how far afield his subjects had traveled. “My hands were shaking so terribly,” he said. “I ran back to the research station and hooked it up to my computer.”
It was a eureka moment: He learned the bird had departed its winter range in Tierra del Fuego and flown some 6,000 nonstop miles until it reached the coast of Texas, its first pit stop on the way back to Churchill. This earned the Hudsonian godwit the silver medal for nonstop long-distance flying, just behind the bar-tailed godwit. It turned out there was a good reason that godwits showed up in far-flung parts of the globe and not in between: They could fly day and night, covering the entirety of Latin America without ever needing to land.
The suite of skills involved in this feat still astonishes Theunis Piersma, Senner’s former mentor at the University of Groningen. Piersma is an expert in the “endurance physiology” that enables shorebirds to execute these migrations. “It’s difficult to pick one thing out because there is this enormous list that is totally amazing,” says Piersma.
On a Zoom call, he ticks off a few items from the list of things a migrating bird has to master: “Distance, time-keeping, navigation, predicting weather, flying at high altitudes, social communication and all of the physiological challenges.” He adds, “Mammals are not the biggest evolutionary success. Birds are better in everything.”
There is a bit of an argument about whether Arctic terns make more impressive migrations than godwits because they fly as far as 25,000 miles round trip, from the Antarctic to the high Arctic. When I bring this up to Senner, he dismisses it with a wave of his hand. “We are always duking it out with those tern people,” he jokes. The difference is that though the tern flies farther overall, it doesn’t complete such long nonstop stretches. It feeds along the way, so it doesn’t need to go through the preflight transformations that the Hudsonian and bar-tailed godwits, and other long-distance migrants, do.
The most critical preparation the godwits make is packing on fat. They do this over a couple of months of gorging on worms, dime-size clams and a variety of other tasty things. The added fat doesn’t negatively affect their performance—it enables it.
Christopher Guglielmo studies endurance physiology at the University of Western Ontario. One of his papers is titled “Obese super athletes: fat-fueled migration in birds and bats.” Guglielmo explains that birds evolved a system to use fat directly—instead of just using sugars or carbohydrates. They burn that fat load up to ten times as efficiently as humans do. This may be the single most important key to their migration success. Oddly, the fat they store in advance also keeps them hydrated. “A marathon runner can grab a cup of water from the sidelines,” Guglielmo says. “Birds don’t have that ability. So when they burn fat they make carbon dioxide and water. It’s called metabolic water. So their fat stores are acting as their canteen.” He adds, “Marathoning is an inadequate metaphor for them. They can’t hit the wall like marathoners do or they’ll die. They don’t drink for over a week. It’s more accurate to say they have a superpower.”
Recent findings also defy the models that researchers have long relied on for the energetic cost of flight to birds, forcing scientists back to the drawing board. “Our models of bird flight say birds should conk out after 3 to 4.5 days,” Guglielmo says. “I am at a loss to explain how they do it for a week or more.”
Unpacking the superpowers of these birds may someday lead to medical breakthroughs for humans. “Understanding how they handle their fat has lots of implications,” says Guglielmo. “Birds look like a Type 2 diabetic. A typical bird has blood sugar concentrations that are way up in the diabetic range. Yet we don’t see any of the diabetic side effects.”
There are other adventures in avian plasticity as godwits prepare for departure. Research suggests that bar-tailed godwits double or triple the size of their pectoral muscles, their heart and their lungs ahead of migration to better power their flight. To compensate, they shrink their gizzards, livers, guts and kidneys. After they arrive at their destination, their bodies readjust. As they head north to breed, the testes of red knot sandpipers grow to 30 times their winter size to supply ample testosterone for singing and other fun things. Some birds grow new neurons ahead of their journeys to supplement the hippocampus, which helps with navigation.
Other adaptations include extremely aerodynamic wings and bodies. Migratory birds sleep while they fly, getting shut-eye on one side of the brain while the other side stays awake and alert, and then switching sides, something called uni-hemispheric slow wave sleep. Dolphins and whales sleep this way as well.
Migratory birds also have extremely efficient respiratory systems, much more so than other animals, which is a good thing because they are often flying at altitudes where there is half as much oxygen as at sea level. Before migration, bar-tailed godwits soup up their circulatory system by increasing the number of red blood cells so they can take more oxygen out of each breath. They store air after it passes through their lungs and then breathe it again.
Among the most remarkable adaptations are the way-finding abilities of migratory birds. Godwits don’t just point themselves south with a compass and take off for New Zealand or Chile. They navigate nimbly across the vastness and vagaries of the Pacific and the Americas. They have a very sophisticated understanding of what lies ahead, and how best to handle it, from low-pressure systems to wind regimes and rain. “They make adjustments we would have a hard time doing with all of our information,” said Piersma. “They play with the winds in very strategic ways. They know weather systems and they understand them. It’s just incredible.”
There are numerous tools birds use to migrate—the stars, landforms, smells—but even adding all of these together seems to get at only a part of the story. Researchers are still looking for a more complete explanation. A magnetic sense has long been known to play a role, but the mechanism hasn’t been proven. One idea holds that there are magnetite crystals in the birds’ cells or their beak that respond to the earth’s magnetism, gently guiding them on course. Or perhaps magnetic forces tug at magnetite in the inner ears of the animals. Another leading hypothesis these days is that bird vision is connected to magnetic lines on the earth’s surface involving an abstruse process called “quantum entanglement”—a phenomenon of physics that Einstein once called “spooky action at a distance.”
Piersma shrugs on the Zoom call when I ask him what the mechanism is. “What is undeniable is that without a strong magnetic sense you just can’t explain a lot of the navigation performance,” he says. “When we follow them with trackers we very often feel as if they have a GPS on board and have the same geographic knowledge we have.”
One of the leading questions in the bird world is how they might communicate among themselves, and the role of the information they share during migration. While just a quarter of young birds survive the deadly gantlet of the Alaskan bog, some 90 percent of adults survive the slings and arrows of their southward migration. That may be because birds that travel in groups form a collective intelligence, a group mind that makes far better decisions than a lone bird.
Migratory shorebirds are not only equipped with highly evolved adaptations for flight and migration; their beaks are sophisticated instruments, too, with an upper bill whose end can flex up independently from the rest of the bill to catch squirming critters. Tiny sensors, called Herbst corpuscles, cluster at the tips of their beaks. These pressure sensors seem to enable a kind of tactile echolocation: With their beaks plunged deep into the mud and sand, the shorebirds can sense the movement of prey.
And then there is the why of migration. As they pursue their eternal summer, the long periods of daylight allow the birds to enjoy more hours of feeding. They also return to the north because high latitudes are extremely rich in foods that shorebirds love. In the Bohai Bay of northern China, for instance, a critical migratory shorebird stopover, there are some 50,000 gem clams in one square meter of mud. In the Arctic regions, there’s an abundance of insects for the young ones when they hatch.
A few weeks before the summer solstice, under a nickel-gray Alaskan sky, Senner makes his nest rounds and finds that the mossy brown, spotted egg shells are being “starred”—the chick is ready to go and its sturdy beak is creating an asterisk-like crack. He holds the egg to his ear and hears a slight tapping.
The next day the impatient babies are pipping, or starting to break through the shell. Senner is getting excited. “Tonight,” he says. “Maybe tomorrow.”
The hatching arrives later than predicted, almost two days late. It’s the first time he’s been here since 2009, Senner says, that none of the eggs has hatched by June 1. Finally, one morning a few days later, his daily rounds find the chicks are starting to hatch. The downy, mottled babies are whistling away. His record remains intact: He has still not missed a hatch.
At the first nest, he sits down and scoops the chicks up, cradling them in his hands and briskly going about his research as the fierce mother godwit alarms from above. Puleo, the graduate student, assists him. He draws a tiny drop of blood, weighs them, measures the leg, head and bill size of each, and places a minuscule VHF radio transmitter on the fluffy back of half the chicks to follow them around the bog. He’ll recapture them once a week for the first four weeks to understand more about their habits and mortality. The transmitters will fall off before the chicks make their first migration. Every chick also gets a small plastic tag on its tiny matchstick bird legs.
The procedures finished, Senner heads to the next nest. He says the godwit parents always return to the nest after his measurements and rearing resumes as usual.
The adult godwits leave here in June or July and arrive in Chiloé Island in September or October, to spend the next several months there and return north in April. The self-sufficient chicks make their maiden migration from Alaska a bit later at the end of July, flying to central Canada on their own, without adult supervision. They may make other portions of their first migration unassisted; it’s not yet known. How they are able to do that is another shorebird mystery. A Hudsonian godwit lives about 10 to 12 years. These new hatchlings will make the global crossing as many as 24 times.
The data gathered here and on the Chilean end will help fill out the unknowns of this bird, and also, researchers hope, offer assistance to a species in decline. Currently there are fewer than 70,000 Hudsonian godwits. The population in Manitoba has fallen some 3.5 percent annually. The Alaska population may also be declining. The bar-tailed godwit population has declined as well in recent years. A big part of the work is answering why.
What Senner is doing here is being replicated in many places. Researchers in Asia, Africa, Europe and the Americas are working in their corners with partners on the other ends of the world. Facing similar issues, they hope to fill in the pieces of the migratory bird puzzle and come up with conservation strategies.
Because these birds travel such long distances to a wide variety of places and many of the problems they face are unknown, there is a long list of disparate challenges. A main culprit in the decline of the Manitoba population of Hudsonian godwits, for instance, is a changing climate that has altered weather patterns and created a mismatch between the time the godwit babies hatch and peak insect abundance. Now, when the chicks emerge from their shells, the bugs are mostly gone, making them susceptible to starvation. The bugs are still plentiful when they hatch here in Alaska, but if their numbers are declining, it may have something to do with their single stopover en route, in the ephemeral wetlands of the central U.S. Precipitation patterns and farming practices are changing, and ponds may be drying up. Senner’s PhD student, Jennifer Linscott, is studying that now.
“Birds aren’t falling out of the sky,” says Senner, “So what is it? I think the problems are compounding. They don’t gain enough weight before they leave, they hit head winds, water isn’t where they found it before, and they arrive in Alaska and it’s warmer than expected. That’s how these declines are happening.”
In 2014 Senner started corresponding with Juan G. Navedo, a Spanish biologist who works with a team of students and biologists in Chile. “Tourism is one of the main threats to godwits outside of their breeding areas,” says Navedo, who heads the Bird Ecology Lab at the Universidad Austral de Chile with support from the Chilean National Science and Technology Research Fund. “There is a lot of shoreline development. It’s a big, big problem.” Hunters kill the birds, feral and domestic dogs chase them, people build shanties in their feeding grounds, locals gather red algae with ox carts and tractors to provide agar for the manufacture of cosmetics and pharmaceuticals, and piles of trash add to the constellation of threats. With financial support from the Packard Foundation and the National Audubon Society, Chile’s Center for the Study and Conservation of Natural Heritage secured an important roosting site on Chiloé Island.
Navedo is also concerned about the chemicals and antibiotics that wash ashore from massive salmon-rearing pens in the Gulf of Ancud. The toxins get absorbed into the sediment, and the birds who live there carry high levels of antibiotic-resistant bacteria, probably ingesting it from the worms they eat. Navedo and his students cast cannon nets over the birds and catch 30 to 40 of them at a time to sample their gut microbiota.
Back on the bog at Beluga, Alaska, there are no concerns about crowds of people. The researchers are the only ones visible in the vast, empty landscape. A Hudsonian godwit mama roosts at the top of a black spruce, watching over Senner as he goes about his business with her babies.
My time here with the godwits is helping me realize that what we see in these magnificent creatures is not only a bird, but the incredible result of the millions of years of sculpting and tweaking by evolutionary pressures. That makes their decline even more poignant, and ominous. “I see them more and more as the Romans did, as augurs, with everything that is happening,” says Piersma. “Civilization is built on the constancy of climate. Listen to the birds for God’s sake and they will tell us what is going on.”