Every November, around the Day of the Dead, millions of monarch butterflies descend on a forest of oyamel firs in the mountains of central Mexico. The butterflies have never seen the forest before, but they know—perhaps through an inner compass—that this is where they belong. They leave Canada and the northeastern United States in late summer and fly for two months, as far as three thousand miles south and west across the continent. The journey is the most evolutionarily advanced migration of any known butterfly, perhaps of any known insect. But climate change and habitat loss, both in the forest (photographed here in February last year) and in the U.S., are fast eroding the monarchs’ numbers.