El Rosario Monarch Butterfly Sanctuary
Twenty million monarch butterflies taking flight sounds like a gentle whispering wind. It looks like a shimmering blizzard of dancing orange-gold motes. It is a natural phenomenon that takes place on south-facing Sierra Madre mountain slopes west of Mexico City every winter and spring.
Perhaps 100 million monarchs from Canada and the U.S. east of the Rocky Mountains spend winters here. They disperse among several sites where ideal conditions of food, temperature, and moisture exist for them in dense oyamel fir woods (monarchs west of the Rockies go to California).
When they cluster at night and on cool days, they can almost cover branches and trunks of trees. When sun warms the air, they fly up and gather to drink at streams and sip nectar from flowers.
The life story of these dazzling, fragile creatures is one of the most remarkable in the natural world. What they do has been known only since 1975. How they do it is still unknown.
Butterflies arriving to winter here have flown up to 3,100 miles (5,000 km), obeying an instinctive directional pull handed down by ancestors which preceded them by five generations. Once here, they become semi-dormant, gradually awakening as northern spring beckons. In March, they mate. Females then fly to Texas to lay eggs, and, five short-lived generations later, their descendants will be in Canada, Montana, Maine—wherever their ancestors came from. That generation will delay sexual development and start fluttering southwest in fall across the U.S. and Canada at average speeds up to 20 miles an hour (33 kph) covering up to 120 miles (200 km) a day, to start the cycle over again.
Scientists have taken Maine and Montana monarchs and exchanged them. All have ended up together here on a Mexican slope they have never seen but whose existence and location they sense unerringly.
Threats to this natural marvel are multiple—logging and habitat loss in Mexico among poor residents who value income from a logged tree more than a butterfly (one of the five protected areas is already seriously damaged); and all along summer and migration paths from loss of milkweed, their only larval food plant, often considered an undesirable weed. Conservation groups such as non-profit Monarca and World Wide Fund for Nature are working to enable Mexicans to benefit from ecotourism, and elsewhere to encourage milkweed-planting.
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Tens of millions of monarch butterflies set out every August on a six-week 3,000-mile (5,000-km) journey from Canada and the northern U.S. to a destination they have never seen in Mexico. There they gather for the winter in El Rosario Sanctuary, covering trees, sometimes flying up from winter naps in a softly-whispering blizzard of black and orange insects. In spring they will reawaken, mate, and their descendants will return to the same places their ancestors came from—exactly how, no one knows—but the next generation will make the same miraculous-seeming journey the following year.
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