“¡Mire! ” My guide, Silvestre, pointed upward, as if I wasn’t already walking with my neck craned as far back as it could go. The sky was alive with millions of fluttering shapes, congregating into clouds at random, occasionally turning into ribbons that wound through the trees. So many butterfly wings beat the air that their vibrations were audible, like a waterfall rushing in the distance. I understood for the first time the immensity of the event I’d come to see.

I was in the center of El Rosario butterfly sanctuary, part of Mexico’s Monarch Butterfly Biosphere Reserve. Unaccustomed to the thin air at nearly 11,000 feet above sea level, I’d ridden Pelucha, a rust-colored mare, to the heights of the reserve. About halfway up, the orange-and-black butterflies had begun appearing in groups, alighting on flowers or gathering to sip water from puddles. Coupled pairs mated everywhere, and their constant entanglements on the path made it difficult to avoid stepping on them; Pelucha happily crunched more than a few underfoot. I dismounted at the top of the trail and followed Silvestre as he ducked under a rope that kept tourists from accessing the most sensitive areas of the reserve.

It was February, and the monarchs—northerners that had wintered in Mexico—were preparing for their final journey, which doubled as the first leg of a multigenerational relay race north. Their delicateness belies a life cycle that more resembles long-haul trucking than anything you might read in a children’s book. Each of the butterflies above me had already braved many grueling trials. In the following week or so, the majority would be on their way.