Monarchs in Mexico - Where Monarch Butterflies Winter

Monarchs in Mexico
Where They Overwinter

Mexico: Monarchs by the Millions

The millions of monarchs headed for Mexico overwinter in an 80-square mile area near Mexico City. All the sites are in relic fir forests in cool and moist mountain areas. The overwinter spots are small, each one only between one and four acres.

Yet a single overwintering site may hold between ten to 50 million monarchs, which literally cover the trees. The butterflies, which individually weigh less than a penny, are sometimes so concentrated they cause the smallest trees to snap. (Where to see Monarchs in Mexico)

Monarchs do not feed while overwintering but, like bears, go into a state of hibernation and rely on the fat they stored during their migratory journey; the overwintering sites themselves have little food to provide. The butterflies will become active again only when the weather begins warming, usually in February.

When such a great portion of the entire North American reproductive population is concentrated in Mexico in only a handful of sites, the species as a whole is very vulnerable to disaster.

Fortunately, only two species of birds have adapted to feed on Mexico's overwintering butterfly colonies, and they reduce the butterfly population by as much as 10 percent each year. It's estimated that marauding mice destroy another 5 percent.

There are greater threats to the monarchs than other animals. Monarchs are present in the high forests during the dry season, when the forest is most susceptible to fire.

A fire that kills off all the butterflies in a single location may devastate as much as 10 percent of the entire North American monarch population.

But the real problem has been freak snow storms, which have occurred several times in recent years. One storm in the 1990's killed an estimated 70 percent of the monarch population.

Monarch Butterfly Life Cycle

Monarchs and Milkweed

Monarch Migration Patterns

Where To See Migrating Monarchs

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Monarchs in Mexico Where They Overwinter

Monarchs in Mexico
Where They Overwinter