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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.
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 was estimated
to have killed 70 percent of the monarch population when the colonies
were again covered with snow in December, 1995.
Monarch
Butterfly Life Cycle
Monarchs
and Milkweed
Monarch
Migration Patterns
Where To See
Migrating Monarchs
Monarch
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