Why monarchs fly 4,000 km to Michoacán's fir forests
Every autumn, monarch butterflies from eastern North America head for a handful of mountains in central Mexico. They spend the winter packed onto fir trees about 3,000 m up, then turn north again in late winter. This page explains the journey in plain terms, with the caveats scientists still attach to it.
Who makes the trip south
The butterflies you see in Michoacán and the State of Mexico are the eastern population of the monarch butterfly. They breed over a wide area of the United States and southern Canada through the summer.
When the days shorten, the last generation of the year stops breeding and starts flying southwest. Some travel up to about 4,000 km (2,500 miles), riding warm air and tailwinds where they can, to reach the mountains west of Mexico City.
Monarchs west of the Rocky Mountains mostly winter along the California coast instead. The Mexican colonies belong to the eastern butterflies, and they gather in a remarkably small patch of forest.
A generation that lives eight months
A summer monarch usually lives a few weeks. It hatches, feeds, mates, lays eggs and dies, and the next generation takes over. Several of these short lives follow one another between spring and late summer.
The butterflies that fly to Mexico are different. They hold off breeding and store fat, and they can live for about eight months, long enough to fly south, sit out the winter and start the return.
None of them has made the trip before. How a first-time traveler finds the same mountains its great-grandparents left is still studied, with the sun's position and the earth's magnetic field among the leading ideas.
Why oyamel firs at 3,000 m
The colonies form in forests of oyamel fir (Abies religiosa) at about 3,000 m (around 10,000 ft). The air there stays cool and moist, which lets the butterflies slow down and spend little of their stored energy.
Too warm, and they would burn through their reserves before spring. Too cold or too wet, and they risk freezing. The dense fir canopy works like a blanket and an umbrella at once, softening frost and rain.
They hang in tight clusters on the trunks and branches, and one tree can carry thousands. On sunny, warmer afternoons many take off to drink at streams and damp ground, which is when the air fills with orange wings.
On cold or cloudy days the clusters stay closed and still. The butterflies are there, but they look like dull brown leaves hanging from the branches. Our guide to the best time to visit covers when the colonies are usually fullest.
How science found the colonies
People in the mountain villages knew the butterflies long before anyone wrote about them. The colonies were described to science in the 1970s, the result of research led by the Canadian scientist Fred Urquhart, who had tagged monarchs for years to trace their route.
The forests now sit inside the Monarch Butterfly Biosphere Reserve, which straddles the border of Michoacán and the State of Mexico about 100 km (60 miles) west-northwest of Mexico City. UNESCO added it to the World Heritage list in 2008.
Only a few parts of the reserve receive visitors. Our page on El Rosario, Sierra Chincua and Piedra Herrada compares the ones our tours visit.
Late winter, mating and the flight north
As the days lengthen toward the end of winter, the colonies stir. The butterflies mate in the forest and then leave the mountains, heading north and northeast toward Texas and beyond.
Spring travel depends on milkweed (Asclepias), the only plant their caterpillars eat. Females lay their eggs on it along the way, and the long-lived travelers die soon after.
Their offspring carry the journey on, and it usually takes several short-lived generations to fill the breeding range again. Then, in late summer, a new long-lived generation is born and the cycle repeats.
Why the numbers rise and fall
Researchers measure the colonies by the area of forest they cover rather than by counting butterflies. That area changes a lot from one winter to the next, and it is never a fixed figure you can plan a trip around.
Several things are thought to drive the swings:
- Weather on the breeding grounds and along the route, including drought and heat
- The amount of milkweed left in farmland and roadsides
- Storms and freezes in the Mexican forest during winter
- The health of the fir forest itself, which logging and dry years can thin
Scientists still debate how much each factor weighs. For a visitor, the practical point is simple. The colonies can hold millions of butterflies, but what you see depends on the year, the week and the weather on the morning you arrive.
Seeing the migration for yourself
Every tour on our site takes you to one of the wintering colonies, and the walk in is part of the experience. At about 3,000 m the air is thin, so read our guide to the climb before you choose.
Sierra Chincua in a small group from Mexico City
Paseos Turisticos drives a small group to Sierra Chincua, allows about five hours for the visit, and ends with a Michoacán lunch at a cooking workshop. It scores 4.8 from 60+ reviews on Viator, but the booking is non-refundable.
The forest is fragile, and the butterflies need every bit of stored energy to survive until spring. Our page on the sanctuary rules explains why guides ask for quiet and keep visitors on the path.
If you want to compare the full day of each option, the comparison table on our home page sets the tours side by side. The cost guide lists what you pay on top of the ticket.