In the eastern-most corner of Michoacán, straddling the border of México state, lies the incredible 563-sq-km Monarch Butterfly Biosphere Reserve, a Unesco World Heritage site since 2008. Every autumn, from late October to early November, millions of monarch butterflies begin to flock to these forested Mexican highlands for their winter hibernation, having flown all the way from the Great Lakes region of the US and Canada, some 4500km away. The butterflies arrive in waves, the last remaining until early to mid-March, when they begin their return journey north.
As the butterflies close in on their destination, they gather in gentle swarms, crossing highways and fluttering up steep mountainsides where they cling together in clusters that weigh down thick branches of the oyamel (Abies religiosa or `sacred fir') trees. When the sun rises and warms the forest, they take to the sky in gold and orange flurries, descending to the humid forest floor for the hottest part of the day. The best time to see them is on a warm, sunny afternoon when they carpet the ground brilliantly – they don’t fly as much in cool weather (55 degrees or lower).
Note that the entrance fee includes a mandatory guide, who works for tips; the minimum recommended amount is M$220 per guide per day.