Monarch butterfly super generation completes epic 3,000-mile migration to Mexican mountains

In the mountains of Michoacan, Mexico, sixty-six million monarch butterflies hang from the branches of fir and pine trees like clusters of dead leaves, their wings folded tight against their bodies. The branches sag under the weight of them—a sight so improbable that it seems to bend the rules of what a single insect, weighing almost nothing, can accomplish in aggregate. These butterflies have traveled three thousand miles from Canada and the northern United States, a journey that takes three months or longer, guided by internal compasses that read the sun's position and the Earth's magnetic field. They arrived here in late October, and they will stay through the winter in this remote forest eleven thousand feet above sea level, waiting for spring to arrive and for the warmth to call them north again.

What makes this migration possible is a biological accident of timing. Most monarch butterflies live about a month. They eat, they mate, they lay eggs, and then they die. But in late summer and early fall, a different generation emerges—what scientists call the super generation. These butterflies sense the shortening days and cooling temperatures and suppress the reproductive hormones that would normally drive them to breed. Instead, they redirect that energy into survival. They can live up to nine months, nine times longer than their siblings. This metabolic shift, called diapause, is what allows them to undertake their epic journey and endure the Mexican winter.

The mystery of how they know where to go remains largely unsolved. Multiple generations of monarchs, each living only weeks or months, somehow carry the same instinct to fly to the exact same mountain range in Mexico—a place most of them have never been. Jorge Rickards, director of the World Wildlife Fund in Mexico, calls it one of the most magical things in the natural world. The monarchs were first discovered here by researchers in 1975. The Mexican government declared the area a federal reserve eleven years later. But for decades, illegal logging threatened to destroy the forest that shelters them. The roosting area once covered forty-five acres. It had shrunk to four and a half.

Conservation efforts have begun to reverse that decline. The World Wildlife Fund has worked with local communities and indigenous groups to protect the forest and help them profit from tourism instead of logging. They've planted trees, measured butterfly colonies, and assessed the health of the forest. The results are becoming visible. This past hibernation season, the butterflies covered more than seven acres of forest—more than double the area from previous years. The population in Mexico had declined seventy percent over the previous three decades, but this year the number of monarchs that survived the journey north nearly doubled.

When the sun warms the mountain in the morning, the butterflies begin to stir. Their muscles are too cold to fly when they first wake, and many fall to the ground during the night. A human hand, cupped around one of these grounded butterflies and warmed by breath, can revive it enough to fly. As the day heats up, flashes of orange appear on the branches. The monarchs open their wings. Then, suddenly, the air fills with them—thousands upon thousands taking flight at once, their wings creating an audible hum, the whole forest seeming to pulse with motion.

But the monarchs face new threats on their journey home. Less than twenty percent of those that depart Mexico actually make it back to the United States and Canada. Worsening storms and rising temperatures kill many along the way. When they do arrive, they face another crisis: the plants they need to lay their eggs on have nearly vanished. Milkweed is the only food source that monarch caterpillars will eat, but it has been virtually eradicated from agricultural land by the mass spraying of herbicides on genetically modified corn and soybean crops. Chip Taylor, an eighty-seven-year-old ecologist who founded Monarch Watch at the University of Kansas thirty years ago, has made it his mission to restore milkweed. His organization has tagged more than two million monarchs to track their migration. They encourage people to plant milkweed in their gardens and on public land, creating what they call Monarch Waystations. Fifty thousand have been planted so far.

The work has not gone unnoticed. Eduardo Rendon, head of the World Wildlife Fund's Monarch Butterfly Program in Mexico, recently found one of Monarch Watch's tags on the forest floor—proof that the butterflies tagged in Kansas and tracked in the United States had made it all the way to Michoacan. It was a small piece of evidence in a much larger story of resilience and recovery. In Mexico, the arrival of the monarchs in late October carries spiritual weight. Many people here believe the butterflies are the returning spirits of family members who have died, arriving to visit their loved ones during the Day of the Dead. Whether that is true or not, the monarchs are returning—and with each year that more of them survive the journey, the mountains of Mexico fill again with wings.