Monarch butterflies, Monarch Butterfly Biosphere Reserve, Angangueo, Mexico

© Sylvain Cordier/Minden Picture

Monarch butterflies

Every year, one of the most spectacular natural phenomena in Mexico takes place in the forested mountains west of Mexico City. Between mid-January and late March, colonies of monarch butterflies migrate here from colder northern climates to find warmth and begin their breeding season. These huge flying colonies may contain as many as 20 million butterflies, and they can travel up to 160 kilometres per day.

When they arrive in the Mexican state of Michoacán, they'll settle into the forests of fir trees like those shown here, before finding their way to milkweed plants to mate and lay their eggs. The eggs will hatch after just a few days, leaving the offspring to eat the milkweed before eventually transforming into the next generation of adult butterflies. Once the winter breeding season is over, the newly hatched monarch butterflies will start the annual migration cycle over again, taking to the air for the long journey back north.