• Image about Mexico

It’s one of Mother Nature’s most beautiful and inspiring mysteries: the migration of the monarch butterfly from North America to the Michoacán forest in central Mexico and back. It’s been happening for thousands of years, and it will, no doubt, continue for thousands more. But will we ever truly understand why?

Photographs by Sean McCormick

TEN thousand feet up a slope in the Sierra Nevada, within central Mexico’s Monarch Butterfly Biosphere Reserve, the dusty trail of the El Rosario butterfly sanctuary abruptly becomes cement steps. Above our heads, bright orange flecks dart in and out of the trees. Each autumn, millions of monarch butterflies migrate from North America to this dormant volcano, swarming across highways and riding high-altitude currents, stopping only to rest in the occasional tree along the way.

After hibernating in the Michoacán forest during the winter, the monarchs awake in the spring, mate, and begin the journey back to the United States and Canada to lay their eggs. It’s an incredible feat of nature. Some will travel more than 5,000 miles. And they’ve been doing this every year for thousands of years.

But the miraculous journey is not without its pitfalls. “Oh, no,” says our guide, Alfredo, as he stops to bend down and gently pick up a wounded mariposa monarca from the trail. It’s a male, almost dead, feebly moving its wings and legs. “See?” he indicates. “The stomach is missing.”

The monarch can be highly toxic, Alfredo explains. Even a cow can die from eating one. The only part that’s not poisonous is the stomach. Unfortunately for the monarch, its predators have learned this. Local birds, in particular orioles and grosbeaks, attack the butterflies in midair, eat what they can, and then let the insects fall to the forest floor, where they wiggle helplessly for a few minutes before dying. It’s a horrendous fate for such a regal creature, but nature is not always pretty.

humans might prefer the beaches of Puerto Vallarta or Ixtapa in Mexico for our winter vacations, but monarch butterflies are much more discerning. They congregate in colonies atop only 12 specific volcanic peaks in central Mexico’s Trans-Mexican Volcanic Belt, a 75-mile-wide protected reserve.

The region produces 80 percent of the world’s avocados, but civic leaders here heartily embrace their other natural resource too. The Morelia school soccer team is named the Monarchs, the mining town of Angangueo hosts its annual Monarch Festival each spring, road signs throughout Ocampo boast cute little butterfly icons, and the footpath beginning at the El Rosario sanctuary’s parking lot is lined with butterfly-trinket vendors.

As we continue our hike up the trail, Alfredo explains that out of the 12 sanctuaries, two are in this region of the Michoacán forest; of those, El Rosario is the most visitor friendly (the other you have to enter on horseback). Approximately 200 million butterflies hibernate in Michoacán each winter, 20 million of which rest in El Rosario.

Each February, the colonies awake and head north to lay their eggs in warmer conditions. By March, the first generation of eggs has been laid in the southern United States. As these eggs hatch, the new butterflies continue the migration, mating as they go, and in April, the Midwest sees a second generation of eggs. A third generation is then born in the Great Lakes region and in the northeastern United States around July and August.

According to Alfredo, it takes the butterflies three generations to reach their final resting place in the United States and about five to get to Canada.

As the temperature begins to drop in September, the final generation of butterflies then starts moving south again, toward Mexico, roosting overnight in trees along the way. Swarms can be spotted in the Midwest states of Oklahoma, Kansas, and Iowa around early October, and then they cross into Mexico, via central and coastal Texas, a week or so later. This hardy final generation doesn’t breed or die along the way -- it stays the course and arrives at the volcanic peaks of Michoacán in November to tuck in for the winter. (Monarchs east of the Rocky Mountains follow this North America–to-Mexico migration route. Butterflies west of the Rockies funnel down to Southern California for their hibernation months.)