The Monarch Butterfly Biosphere Reserve covers 56,000 hectares of mountain forest straddling the states of Michoacán and México. From late October through March, the reserve hosts an estimated 100–300 million monarch butterflies that have migrated from Canada and the northern United States to spend the winter in the oyamel fir forests at altitudes between 2,400 and 3,600 metres.
The El Rosario sanctuary is the most accessible and largest of the colonies open to visitors. You hike uphill (or ride on a local horse) through increasingly dense forest until you reach the winter cluster — a vast orange and black canopy covering the trees in every direction. When the sun warms the hillside in mid-morning, millions of butterflies take flight simultaneously. The sound is remarkable: a soft rushing, like rain, as their wings beat the air.
The sanctuary is open daily during the season. Entry fees go partly to the local ejido communities who protect the forest. Hire a local guide at the entrance — they know where the main clusters are, which changes from year to year. Dress warmly; the altitude and shade make it cold even when the valley below is warm.