Monarch butterflies in the Monarch Butterfly Biosphere Reserve, Angangueo, Mexico

© Sylvain Cordier/Minden Picture

King of the butterflies. Monarch butterflies in Angangueo, Mexico

Every year, one of the most spectacular natural phenomena in Mexico takes place in the forested mountains west of Mexico City. During the fall, colonies of monarch butterflies migrate here from colder northern climates to find warmth and begin their breeding season. These huge flying colonies can contain as many as 20 million monarchs, which make use of air currents to travel as far as 100 nautical miles per day. The monarchs remain in Mexico from around November to March.

Upon arrival in the Mexican state of Michoacán, they settle into the forests of fir trees like those shown here. Then they begin finding their way to milkweed plants to mate and lay their eggs. The eggs hatch after just a few days, leaving the offspring to feast on the milkweed before eventually transforming into the next generation of adult butterflies. Once the winter breeding season is over, the newly hatched monarchs start the annual migration cycle over again, taking to the air for the long trek back north.