Millions of monarchs wing their way to Mexico for the cold months. This family visited the butterflies’ winter grounds to learn what the flap’s all about.

We stood in a grove of fir trees in the El Rosario Sanctuary, all of us too mesmerized to move. Monarchs—some of them having flown 2,400 miles to this preserve in the Sierra Madre Occidental, 2½ hours west of Mexico City—dripped in clumps from overhead branches, covered trees with pulsating cloaks of orange and onyx, and landed on our noses. The beating of wings sounded like the rumble of a distant waterfall.

Our family had made this weekend pilgrimage to see the mysterious monarchs, which inhabit large swaths of the central and eastern U.S. and Canada during the summer. The insects generally live two to six weeks, but, come fall, for reasons scientists don’t fully understand, the season’s last brood survives up to eight months—long enough to make its way on paper-thin wings to the mountains of Michoacán. Here, the butterflies hibernate and mate before the females head north again to keep the cycle going.

Since 1986, Mexico has set aside 279 square miles of forest to protect the butterflies’ wintering spot from logging. Of the two preserves that are open to the public, El Rosario is the better known, with a walking path that leads right into the thick of things. An hour away, at Sierra Chincua Reserve, visitors can rent horses for the mile-or-so trek into the viewing areas.

Our base was the cobblestoned mining town of Angangueo, the unofficial capital of monarch country. In our hotel room our eight-year-old, Jeb, made like a monarch, zipping himself into a sleeping bag. "I dreamed about the butterflies," he told us in the morning as he peeled off his cocoon. "And I was king of them all!"

February and March are the best months for butterfly-watching at El Rosario Sanctuary and Sierra Chincua Reserve. The Michoacán section of has information about the migration, as well as lodging options.