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Most monarch butterflies that live east of the Rockies—up to one billion individuals—migrate to just 12 cool, high-altitude mountain forests in central Mexico every winter. The oyamel fir trees there act as a blanket and an umbrella, keeping the butterflies at a comfortable temperature and humidity until they leave in March to return north. The butterflies overwinter at these sites year after year, a migratory phenomenon unique among insects.

Recent images from the high-resolution IKONOS satellite show illegal clear-cutting of oyamel firs in a protected area of Lomas de Aparicio, one of the overwintering sites that had large colonies of monarchs last year.

“This is one more example of serious deforestation events that go back to 1986,” says Lincoln Brower, biologist and monarch expert at Virginia’s Sweet Briar College. “If the butterflies lose the safe overwintering areas in Mexico, they will freeze to death when they return to the cleared forests. All that will remain will be very small populations in deep southern Florida, with occasional individuals that disperse northward.”