Imagine an animal that weighs less than a paperclip, has a brain the size of a pinhead, and has never made a journey before. Now imagine that animal flying nearly 3,000 miles, alone, without a parent, a flock, or a map, navigating through storms and deserts and wind, to land on a single mountainside in central Mexico that it has never seen, in a tree its grandparents never reached.
This is the monarch butterfly migration, and it is one of the most extraordinary biological feats on the planet. To stand among the millions of monarchs clustered in the oyamel firs of Michoacán in midwinter is to witness the end of one of the strangest, longest, and most miraculous journeys in the natural world.
Here is what scientists have learned about how, and why, this remarkable migration happens.
The longest insect migration on Earth
Each fall, the eastern monarch butterfly population (Danaus plexippus), which spends the summer breeding across the eastern United States and southern Canada, undertakes a southward migration of up to 3,000 miles, ending in the high-altitude Oyamel Fir forests of central Mexico.
It is the longest known migration of any insect on Earth.
By late October and early November, the monarchs begin to arrive in Mexico, settling in dense clusters on the oyamel trees of Michoacán and the State of Mexico. They spend the winter there, in a kind of energy-saving stupor, then begin the return journey north in February and March.
Western monarchs, a separate population, have their own much shorter migration along the Pacific coast of California, but the eastern population’s journey to Mexico is the one that has captured imaginations worldwide.
A migration that takes four generations
Here is one of the most extraordinary things about the monarch migration. No single butterfly makes the round trip.
The monarchs that arrive in Mexico each fall are typically the great-great-grandchildren of the monarchs that left Mexico the previous spring. The migration is intergenerational, a kind of relay race that takes four or even five generations to complete.
Most monarchs live just two to six weeks. They emerge from the chrysalis, breed, lay eggs, and die. As the population moves north through the United States and Canada in spring and summer, generation after generation hatches, breeds, and is replaced.
But then comes the fourth or fifth generation, born in late summer in the northern reaches of the migration range. These butterflies are different. They emerge in a state of reproductive diapause, meaning they do not mate or lay eggs. Their bodies are built for endurance rather than reproduction. They live not the typical few weeks, but eight to nine months. Scientists have nicknamed them the “Methuselah generation” or “super generation.”
It is this generation that flies the entire 3,000-mile journey to Mexico, overwinters in the oyamel forests, and finally begins the journey back north in February. Most die along the way, but not before reproducing in the southern United States and passing the migration along to the next generation, which continues the journey north.
How do they find their way?
How a fragile butterfly with no prior experience navigates 3,000 miles to a precise mountainside in Mexico has been one of the great open questions in animal navigation. Decades of research have begun to reveal the answer.
Monarchs use a sun compass. Specialized photoreceptors in their eyes track the sun’s position, while a circadian clock in their antennae compensates for the sun’s movement across the sky. Together these systems allow the butterfly to maintain a consistent southwesterly heading throughout the day.
Monarchs also appear to use the Earth’s magnetic field as a backup. On overcast days, when the sun cannot be seen, magnetic sensors in the butterfly’s antennae take over and provide a directional cue. This combination of sun and magnetic navigation appears to be remarkably reliable, allowing monarchs to reach the same mountainsides in Mexico their ancestors used, year after year, generation after generation.
How the butterflies recognize the specific oyamel forests when they arrive is still not fully understood. The mountains are tall, the climate is precise, and something about that location, perhaps a combination of altitude, humidity, temperature, and tree species, signals to the butterflies that they have arrived.
A discovery in 1975
The wintering grounds of the eastern monarch were unknown to science until 1975. Canadian zoologists Fred Urquhart and his wife Norah had spent decades tagging monarchs in Ontario in an effort to track where they went each fall. Volunteers across North America joined the effort, attaching tiny paper tags to butterflies’ wings.
The breakthrough came when a citizen scientist in Mexico named Ken Brugger, working with the Urquharts, finally discovered the wintering colonies high in the mountains of Michoacán. The discovery was published in National Geographic in 1976 and made the cover of the magazine.
What the Urquharts found defied imagination. Tens of millions of monarchs, hanging in dense clusters from oyamel firs, the trees themselves bending under their weight. The image of a butterfly so light that you can hold a hundred in your hand, gathered in colonies of hundreds of millions, remains one of the most arresting in the natural world.
A species under pressure
The eastern monarch population has declined dramatically over the past three decades. Estimates from the 1990s placed the wintering population at close to one billion butterflies. Recent winters have seen colony sizes a fraction of that, with some years recording fewer than 50 million monarchs in Mexico.
The reasons are well documented. The widespread loss of milkweed, the only plant on which monarch caterpillars can feed, due to herbicide use across American agricultural land. Climate change, which is altering the timing and conditions of migration. Illegal logging in the Mexican wintering forests. Pesticide use along the migration route.
In July 2022, the IUCN added the migratory monarch butterfly to its Red List as endangered.
The good news is that monarchs have become a flagship species for conservation. Milkweed planting initiatives, monarch-friendly gardens, and protected wintering forests in Mexico are all part of a coordinated effort to bring the migration back. Visiting and supporting the Mexican sanctuaries is one of the most direct ways travelers can help.
A connection to the Day of the Dead
For the indigenous Mazahua and Otomí people of central Mexico, the arrival of the monarchs each year carries spiritual weight. The butterflies typically arrive in late October and early November, around the time of Day of the Dead. According to long-standing tradition, the monarchs are believed to carry the spirits of the departed back to their families for the holiday.
Whether one accepts the spiritual interpretation or not, the timing is uncannily perfect.
Witness the journey at its end
A Closer Look Tours’ upcoming Monarch Butterflies in Mexico journey is built around private access to the most spectacular sanctuaries, expert local naturalists, and the kind of unhurried itinerary that gives you real time among the trees on the days the colonies are most active.
