The sheer size of the congregation of monarch butterflies in the hills of Michoacán is astonishing, but not as impressive as their 4500km migration. In the autumn, when the weather starts to turn cold in the Great Lakes region of the US and Canada, the butterflies head south, taking just four to five weeks to make it to Michoacán. Here, in an area of less than 150 square kilometres, they find the unique microclimate a perfect place to spend the winter. The cool temperatures allow them to conserve energy, the trees provide shelter from wind and precipitation and the fog-laden air prevents them from drying out. Monarchs typically have a life cycle of around two to five weeks, but when they fly south they go into a phase known as “reproductive diapause”, and end up living for seven months in all. The same butterflies remain in Michoacán all winter, then breed in spring in time for their caterpillars to dine on the newly emergent milkweed plants – their only food source – before returning to the US and Canada.
Around ten percent of all migrating monarchs get eaten by black-headed grosbeaks and black-backed orioles, but that offers no danger to species survival. The real threat is loss of this crucial mountain habitat. This was recognized as far back as 1986, when several key overwintering sites were protected from logging, but the local peasant families need the wood and they were never fully compensated for the loss of this resource. The Mexican government more than tripled the size of the reserves in 2000, but logging continued to a large enough extent that in early 2007 new president Felipe Calderón declared a “zero tolerance” policy against it, and increased policing. To learn more, check out the Monarch Watch website at wmonarchwatch.org.