Monarch Butterflies in Michoacán: the 4,023-kilometre Migration Mystery

At dawn in the mountains of Michoacán, silence is never entirely still. Millions of orange-and-black wings quiver among the oyamel firs, the tall high-altitude trees where monarch butterflies spend the winter huddled together against the cold. A shaft of sunlight slips through the branches and, suddenly, part of the forest takes flight. Mexico has been working to safeguard this one-of-a-kind refuge since the 1980s. Yet, forty-five years after the first conservation measures, biologists remain preoccupied by the same question: how does an insect, sometimes born more than 4,023 kilometres away, locate this very small group of mountains without ever having been there? The journey seems implausible. And it begins again every year.

In the forests of Michoacán, a spectacle both vast and vulnerable

For visitors arriving at the Monarch Butterfly Biosphere Reserve, the first impression is almost physical. Tree trunks appear to be coated in living bark: in fact, they carry clusters of sleeping butterflies. As temperatures rise slightly, the branches loosen and the sky turns ember-coloured. We all know the feeling of a landscape making us pause, without quite knowing what to say. Here, that landscape relies on an animal weighing less than a gram, yet its arrival supports entire villages, guides, forest rangers and an extensive local conservation network.

Mexican conservation work began to take shape in the early 1980s, before protected areas were formally established in 1986 and the present biosphere reserve was created in 2000. The sanctuary now extends across more than 56,000 hectares in the states of Michoacán and Mexico, although the areas where monarchs actually congregate are extremely small. During the winter of 2023–2024, the area they occupied was estimated at around 0.9 hectares, according to monitoring by WWF-Mexico and Conanp. That figure may sound abstract. Above all, it means that a continental phenomenon can sometimes fit into the equivalent of just a few football pitches.

Forest protection has curbed some illegal logging, with clear results in the areas monitored by local communities. But the challenge does not end at the edge of the fir trees: monarchs also rely on milkweeds, the plants on which their caterpillars feed, throughout their North American route. A butterfly does not recognise borders drawn on a map. Pesticides, droughts, storms and the loss of grassland all take their toll, so protecting Mexico’s final refuge is not always enough to preserve the journey itself.

The 4,023-kilometre mystery: memory without a map

Helping monarchs can begin with something remarkably ordinary: allowing native plants to grow. In a garden, on a suitable balcony or alongside a field, native milkweeds provide food for caterpillars, while nectar-rich flowers sustain adult butterflies along their route. It is better to select local species from a reputable nursery than to buy a packet of seeds with no clear provenance. Let us be honest: nobody turns their outdoor space into a perfect sanctuary every day. Even a few square metres that are mown less often can make a difference.

A common mistake is to assume that every butterfly spotted needs to be “saved” by bringing it indoors, or that any milkweed will do. The instinct comes from good intentions and is easy to understand: seeing a damaged wing creates an urge to act immediately. However, poorly managed home rearing can encourage disease, while certain non-native milkweeds can disrupt migratory cycles in warmer regions. The most useful action is often the least dramatic: avoid insecticides, vary flowering plants and allow a little living untidiness in the landscape.

At the centre of the puzzle is a particular generation, known as the “Methuselah generation”, which emerges at the end of summer. It survives for several months, far longer than summer generations, then travels south to Mexico from Canada and the northern United States, covering up to 4,023 kilometres. It has never seen the oyamel firs or followed its parents, which died long before.

“Monarchs do not simply migrate south: they are heading for a precise place, in mountains they have never known.”

• The sun provides a reference point through an internal compass located in their antennae.
• A biological clock adjusts for the sun’s position as the day progresses.
• Earth’s magnetic field may play a part when skies are overcast, although its precise role remains debated.
• Scents, winds and terrain also appear to offer clues that are not yet fully understood.

Monarch butterfly migration reaches far beyond Mexico’s borders

Researchers have shown that monarchs combine a sun compass with a circadian clock, but this finding does not close the case. A compass provides direction; it does not explain how they target the same forested valleys, at a highly specific altitude, after weeks in flight and thousands of kilometres of turbulence. The insects must also know when to leave, when to conserve energy and where to find nectar. Even a small shift in weather can overturn this finely tuned timetable. Ultimately, the mystery is not that they travel south. It is that they seem able to identify a destination.

Ultralight tracking tags and citizen-observation networks now make it possible to follow part of this odyssey. Researchers record appearance dates and sightings at schools, gardens and fields, then compare the information with weather maps. Every observation illuminates one piece of the story, but seldom the whole picture. Monarchs do not fly in straight lines, nor do they all use the same corridor: they climb, descend and sometimes wait for favourable winds. A storm in Texas or a drought in the Midwest can alter the route of millions of individuals, without anyone being able to predict it fully.

What Mexico has protected for almost forty-five years is therefore not a postcard landscape, but the final chapter of a story written across three countries. When a Michoacán resident watches over a stand of oyamel firs, that work connects with that of a farmer reducing herbicide use in Ontario, or a family allowing its garden to bloom in Minnesota. Biologists are still searching for the exact formula linking the sun, genes, magnetism and landscapes. There is something unsettling about that uncertainty, but also something valuable: it reminds us that a familiar creature can hold a secret greater than our technologies.