Green and his fellow researchers have set up temporary quarters inside one of the area’s many private hunting lodges, and its walls are lined with the taxidermied heads of native and exotic game animals. But Green, a professor of ecology and evolutionary biology at the University of Michigan and a National Geographic Explorer, has eyes only for the three dozen monarchs he captured earlier in the day. He applies a dot of epoxy between the wings of the butterfly in his hand, then affixes a custom-designed sensor—a stack of computer chips powered by a miniature solar panel that together weigh less than three grains of rice. The soft flutter of wings is the only sound in the room.

This monarch and its companions, Green and his collaborators expect, will carry the sensors to the mountains of central Mexico, 800 miles south. In a few weeks, the researchers will follow the monarchs to Mexico, where they will try to detect the signals emitted by the sensors’ antennas. If they can recapture one or more of the butterflies—a big if—they will be able to access the light and temperature data collected by the sensors en route, allowing them to map each butterfly’s path.

Like other monarch research projects across North America, this one has been aided by volunteers eager to help the species. Green’s colleagues, realizing that bicyclists travel at about the same speed as monarchs on the move, recruited cyclists to test the accuracy of the sensors by carrying them on multiday rides. Green conducted laboratory experiments to confirm that the sensors don’t interfere with flight. Now, this novel technology is about to undergo its first real-world test.

When he finishes attaching the sensors, Green sits back in an overstuffed leather chair, surveying the butterflies in the net cage before him. “This year, we’ll be happy if we pick up any kind of signal in Mexico,” he says. Collecting meaningful data might require several more seasons of trial and error, but Green is patient. Smiling, he resorts to scientific understatement: “It’s a real opportunity to understand this particular system.”

As the day cools, Green carries the cage of butterflies outside, picking his way downhill to the pecan grove below the lodge. There, beside a creek, hundreds of migrating monarchs swirl through the lengthening light. Green extracts the sensor-carrying butterflies one by one, gingerly settling them on low-hanging branches like so many glass ornaments. Tomorrow morning, if all goes well, they will continue to venture south, taking their secrets with them.

The system that so fascinates Green is one of the most epic, and dangerous, journeys on the planet. Though monarchs live throughout the world—in South America, the Caribbean, Australia, Europe, and elsewhere—North American monarchs are distinguished by their extraordinarily ambitious seasonal migrations. Each fall, monarchs in the northern United States and southern Canada fly south, the first relay team along a 3,000-mile route known only to earlier generations. Those that survive gather in central Mexico, where they spend the winter in the same fir groves that sheltered their grandparents and great-grandparents the previous year.

Despite decades of study, this annual ultramarathon—and the shorter migration of the continent’s western population along the Pacific Coast—is only partly understood and ever more perilous. Due to climate change and habitat loss, monarchs on both migration routes are increasingly beset by extreme weather and scarce nectar sources. At the same time, the milkweed plants that breeding monarchs need to host their eggs and feed their caterpillars remain in critically short supply, diminishing overall numbers.

The prospects for North American monarchs are considered so dire that the International Union for Conservation of Nature (IUCN) has classified the two populations as vulnerable. They’re now under consideration for protection by the U.S. Endangered Species Act. Those who have witnessed the populations’ decline hope their new status will lead to sustained, multinational action: Karen Oberhauser, who has studied monarchs since the 1990s and recently retired as director of the University of Wisconsin–Madison Arboretum, says that since the monarch was first proposed for protection under U.S. law in 2014, the species has gained new support from government agencies and scientists. “The level of federal engagement has just skyrocketed, and that’s been so important,” she notes. “It’s brought a lot of really smart people into our circles.

While the monarch is neither the largest nor the showiest butterfly in North America, no other insect—and very few species of any kind—so captivates us. Its travels connect people across generations, national borders, and even, it is said, the barrier between life and death. Some Mexican observers of the annual Day of the Dead regard migrating monarchs as souls on the wing. Emergency workers in lower Manhattan during the days after September 11, 2001, saw the monarchs that sailed over ground zero as symbols of survival and rebirth. “When we say that this butterfly is ‘iconic,’ it is exactly that,” says anthropologist Columba González-Duarte of the New School for Social Research in New York City. “It has a place now, for North Americans, as that insect that goes beyond borders, that is capable of the impossible.”

Long before anyone understood how far North American monarchs travel, people celebrated their periodic appearances. Mexican poet and novelist Homero Aridjis, whose memoir recalls his childhood in the central Mexican state of Michoacán during the 1940s and ’50s, wrote that the autumn wind “bore currents of butterflies.” Aridjis and his friends would trek to a nearby mountain meadow to watch the butterflies alight in the firs, captivated by the spectacle.

In the 1950s, Canadian zoologist Fred Urquhart and his wife, Norah, founded the Insect Migration Association, beginning a long tradition of public participation in monarch research. Over the next several decades, the association recruited some 3,000 volunteers to capture individual butterflies and mark each with a tiny label reading “Send to Zoology University Toronto Canada.” From the resulting data, the Urquharts surmised that monarchs spent the winter in Mexico, but didn’t know where. In 1973, when they placed a call for volunteers in a Mexico City newspaper, Kenneth Brugger, an American expatriate, responded. Brugger’s wife, Cathy, now Catalina Aguado Trail, had been paying close attention to monarchs and other butterflies since her childhood in Michoacán. She agreed to lend her language skills and knowledge of the region to the search for the monarch’s wintering grounds.

For two years, first on weekends and then full-time, the couple crisscrossed the mountains of central Mexico by motorbike and on foot. On the afternoon of January 2, 1975, while climbing a volcanic peak called Cerro Pelón, Trail looked up into the firs and stopped short: The trunks and branches above her were covered with thousands of monarchs, so closely packed that their wings overlapped. When Brugger joined her, they both stood silently, awestruck.

Trail and Brugger’s elation soon turned to worry. The monarch’s winter habitat in Mexico is almost entirely limited to 10 or so small patches of high-elevation oyamel fir forest within an area of 217 square miles. In the 1970s, the local communities that hold communal rights to the forests depended on logging for a living, and the evergreen canopy that protects the monarchs from winter weather was shrinking fast. Crowds of curious visitors could further disrupt the habitat.

As word got out, tourists did travel to the mountains to gaze up at the monarchs. But the news also prompted action. The IUCN called on the Mexican government to protect the fir groves, as did the Mexican environmental group Pro-Monarca. Though the government established a national reserve that in October 1986 banned or limited logging in five of the known wintering grounds, the hoped-for economic benefits of tourism for local communities were spotty, and logging continued.

In 2000, after long and sometimes acrimonious debate among government officials, scientists, conservation advocates, and community representatives, the reserve was expanded threefold to encompass most of the monarch’s known wintering habitat. The Monarch Fund—which is administered by the Mexican government and supported by international conservation groups—began making modest but consistent payments to the residents who hold rights within the core zone of the reserve, partially compensating for lost timber income and successful protection efforts. Around the same time, a group of Mexican sustainable-development advocates founded the organization Alternare, which works with communities near the reserve on projects like reforestation and water conservation.

Thanks to these and other initiatives, logging in the reserve began to decline, and by the early 2010s, annual forest loss had fallen from hundreds of acres to single digits—a major conservation success. Since 2019, forest loss has once again increased, this time because of drought-driven bark beetle outbreaks and the legal logging that is intended to control them. Part of the problem, says geographer Isabel Ramírez of the National Autonomous University of Mexico, is that state forest management policies haven’t caught up with the changing climate.

(Can monarchs adapt to a rapidly changing world?)

Early on a December morning, I follow André Green and his team along a narrow trail into the Sierra Chincua monarch sanctuary in central Mexico. My first impression is that the tall, slender trees around us are covered with rusty foliage. When my eyes and brain catch up to reality, I realize that every fir in sight is draped with slumbering butterflies, wings folded to display their paler undersides. The layers of insects are heavy enough to bend even the sturdiest branches. The cool mountain air seems to vibrate, stirred by the countless wings twitching above our heads.

As the struggle to protect the wintering grounds unfolded in the 1990s and early 2000s, scientists from Mexico to Canada were working to understand the monarch’s astonishing annual journey. Longtime monarch researcher Lincoln Brower and his colleagues learned that while the monarchs that overwinter in Mexico travel north in the spring, they don’t complete the trip; they instead lay eggs in northern Mexico and throughout the southern U.S. When those offspring mature, they continue to the northern U.S. and southern Canada, also laying eggs along the way. During the summer, two or three more generations emerge. The final generation, unlike its predecessors, doesn’t immediately reproduce but enters a state of suspended maturation called diapause. When the days begin to shorten and cool, these aging teenagers head south, returning to Mexico in a single generation.

Since these Mexico-bound monarchs can’t ask their great-grandparents for directions to the winter colonies, scientists reasoned that they must be able to navigate. Through a succession of studies, researchers learned that monarchs are equipped with two compasses: a primary system that uses the sun and a backup system that uses the Earth’s magnetic field.

In a study published in 2009, biologist Christine Merlin and her collaborators found that monarchs use circadian clocks located in their antennae to correct their sun-compass readings for the planet’s daily rotation. While this elaborate system keeps monarchs headed in the right direction, it doesn’t fully explain their ability to home in on the same circumscribed wintering grounds year after year.

In the Sierra Chincua sanctuary, the sun climbs higher above the horizon and the rustling in the trees increases. The monarchs open their wings to bask, warming their muscles in preparation for flight, and the whole forest seems to brighten. On the steep slope above the trail, one of the team’s radio receivers stands ready to detect a sensor-carrying butterfly, just in case one has not only reached Mexico but chosen this stand of firs as a winter home. A few monarchs begin flitting from tree to tree, and soon we’re surrounded by a muffled cacophony of millions of moving wings, a torrent that glows above and around us. Some of the monarchs stream out of the grove, while others weave through the trees, radiant in the filtered sunlight, occasionally dipping low enough to skim our faces and hands. All the while, though, the receiver remains silent.

Since returning to their laboratories at the Universities of Michigan, Delaware, and Pittsburgh, Green and his colleagues have improved the energy efficiency of the sensors, ensuring that the solar panels will be able to harvest enough sunlight in the shady reserve forests. In October 2023, they attached 175 sensors to butterflies in Texas, boosting the chances of capturing a signal when they climb into the Sierra Chincua this winter.

“It’s just a marvelous, marvelous organism, and understanding how it’s able to do what it can do allows us to understand the biological world a little better,” says Green. “So as long as they’re performing the behavior, I’ll be interested in overcoming the obstacles to understand it.”

Why protect the migration of North American monarchs? The answers, I found as I followed their journey, are almost as varied as monarch allies themselves. Some, like Green, are drawn to the butterfly’s mysteries; others admire its beauty and tenacity. Many monarch volunteers form international friendships that they come to value almost as much as the butterflies.

For Jane Breckinridge, co-founder of the Tribal Alliance for Pollinators, restoring monarch habitat is part of a broader endeavor to support species of all kinds, including humans. “Monarchs are special and magical, and I love them,” she says. “But the problems they face are the problems faced by all our native pollinators and all our other native critters.”

(Anyone can help monarch butterflies. All you need is a yard.)

A citizen of the Muscogee Nation in northeastern Oklahoma, Breckinridge grew up in nearby Tulsa, then spent two decades in Minnesota before returning to live on her grandmother’s land in 2004. There, she and her husband, David, opened a commercial butterfly farm, cultivating an array of species for sale to zoos and museums. She started a program called Natives Raising Natives, which recruits tribal members to rear butterflies—and the native plants they need—at home for extra income. In 2014, Breckinridge asked University of Kansas professor Chip Taylor for help in restoring a monarch migration corridor on tribal lands in Oklahoma. Taylor, the founder of the volunteer monarch-tracking organization Monarch Watch, was enthusiastic; he knew monarchs badly needed more habitat in Oklahoma. But he suspected it would not be easy: Patches of native prairie are so rare that locally adapted seed supplies can only be acquired through labor-intensive collection.

Ten years later, the Tribal Alliance for Pollinators is the largest producer of native plants and seeds in Oklahoma, and it works with tribes throughout the Great Plains and beyond. Seeds from 230 native prairie species are available free to all tribal members, and each year the small staff distributes tens of thousands of young plants to individuals and institutions. Tribes throughout Oklahoma have documented breeding monarchs making heavy use of the milkweeds in their pollinator gardens, which also host native bees, small mammals, and other species. The plants benefit humans, too, for some have ceremonial significance or medicinal uses, and all are appreciated for their colorful variety.

The Muscogee Nation is home to two distinct Indigenous languages, Muscogee and Yuchi, and the latter is spoken by a few dozen people. That number, however, is growing: At the local immersion school, preschoolers and elementary students greet the day in Yuchi, then tend to the school’s garden of native plants. During the winter of 2019, inspired in part by the work of the Tribal Alliance for Pollinators, the school’s staff and instructors crowded into a van and drove to central Mexico, where they trekked into one of the sanctuaries to see the butterflies that would, before long, be flying toward the Muscogee Nation.

For school co-founder Halay Turning Heart, who learned Yuchi as a child and now speaks it with her young children, the connection between her work and the monarch’s migration is obvious. “We see the language as essential for our survival, and we know the butterflies are struggling for their survival,” she says. “We recognize that they’re fighting for their habitat and that we’re helping to bring it back.” The sight of the monarchs assembled in Mexico, she remembers, was both awe-inspiring and heart-wrenching: “We knew we might not see it again in the same way. We know how quickly things can change.”

Nowhere is the enormous challenge of bringing back monarch habitat more obvious than in Iowa. The state’s fertile soil grows more than two billion bushels of corn each year, almost all of which is used for livestock feed or ethanol production, and rural Iowa is dominated by field after field of corn and soybeans. In 1996, when Monsanto began introducing genetically modified crops that are resistant to the herbicide glyphosate, or Roundup, farms across the Midwest started using glyphosate for weed control, killing milkweed and other benign natives in the process.

Today, native plants find refuge on the grounds of the Tallgrass Prairie Center at the University of Northern Iowa, where staff members tend neat rows of milkweed, spiderwort, and other species. The seeds for these plantings are collected in the state’s vanishingly small remnants of native prairie, several located in the 19th-century cemeteries that were among the few places off-limits to settler plows. Every year, commercial seed, some produced from this genetically diverse stock, is distributed to the state’s county road departments, which plant it on Iowa’s road margins.

The program, started more than 30 years ago as an attempt to sustainably manage roadside vegetation, is now one of the most extensive habitat restoration efforts in the state. Iowa transportation officials estimate that about a quarter of the state’s roadsides are planted with native grasses and wildflowers, and the plants frequently host monarchs and other insects. Throughout rural Iowa, county roadside managers serve as informal ambassadors for the value of native prairie, explaining that what may look like a stand of weeds in need of mowing is a low-maintenance, ecologically rich echo of prairies past.

But Iowa’s roadsides cover only a tiny part of the state, and experts say that in order to reproduce at rates that will stave off extinction, North American monarchs require at least twice the amount of milkweed currently available in the entire Midwest—as well as reliable supplies of other nectar-producing natives along their migratory routes. “We need much more than one percent of the land area to counteract all that’s been lost,” says Laura Jackson, the director of the Tallgrass Prairie Center. The state has little public land—state-managed roadsides make up less than a fifth of it—so Jackson and the rest of the center’s staff also work on private-land habitat restoration through the federal Conservation Reserve Program, which contracts farmland for conservation purposes.

In 2018, they began collaborating on a major restoration project with Cathy Irvine, a retired special-education teacher in northeastern Iowa, who has donated almost 300 acres of her family’s corn and bean fields to the center as a tribute to her late husband, David. On a June afternoon, a pair of monarchs flits across 80 acres already thick with milkweed, wild indigo, purple coneflower, and other native flowers and grasses. “None of this will be planted again except as prairie,” she says with satisfaction.

If Iowa embodies the difficulty of prairie restoration, the Irvine Prairie illustrates its possibilities. Getting the right combination of viable native seeds into hospitable ground is no small feat, and restoration practitioners are well acquainted with failure. “Humility is big around here,” says Jackson. Plantings also require periodic burning or mowing to keep out woody species. Once prairie plants take root, however, valuable habitat can happen fast. Irvine and Jackson look forward to the day when seeds from these native species begin to sprout on nearby roadsides, setting forth into the landscape their ancestors called home.

Each spring, the overwintering generation of monarchs in Mexico performs a final spectacular feat, flying hundreds of miles north to lay their eggs. During an April visit to the Muscogee Nation, I watch a single monarch, identifiable as female by her thick black wing veins, travel low over a sunbaked putting green, the ragged edges of her wings a testament to her endurance. If she hasn’t finished laying her eggs—several hundred in total, typically deposited one by one on the undersides of milkweed leaves—she will soon, for her life is nearly at an end. Her progeny, and theirs, will complete the trip north, flying as far as southern Canada.

This defunct golf course, acquired by the Muscogee Nation from a private owner, doesn’t look much like butterfly habitat, but the monarch is snacking on nectar from a cluster of native plants, and more flowers will bloom soon. Collin Spriggs, a conservation botanist with the Tribal Alliance for Pollinators, parks his blue hatchback on the turf and unloads fragrant trays of lemon bee balm and mountain mint seedlings. Leading the small crew of planters is Muscogee Nation wildlife technician Brooklyn Bartling, whose left bicep is tattooed with images of a blackberry, a bee, and a ladybug.

Bartling excitedly describes the nation’s plan to turn the course into a nature reserve, as well as her work helping to remove invasive plants from the grounds and establish native wildflowers. “I’m taking pictures of butterflies, caterpillars, bugs—everything I see here,” she says. “I want to get that information to the public, to put the why behind what we’re doing.”

She and Spriggs stand over a compass plant, a native perennial they’ve been keeping an eye on since planting it last year. Though it has just two leaves, Spriggs says, its taproot may already extend several feet into the subsoil, affording the plant access to water even during the current drought.

Bartling grins at the news and looks up to survey the fairway. “There’s a lot of potential here,” she says. “A lot of potential.”