The Monarch butterfly (scientific name: Danaus plexippus) is perhaps the best known of all North American butterflies. It is easily recognizable by its bright orange-red wings, with black veins and white spots along the edges. The Monarch butterfly is famous for its southward migration from Canada to Mexico and the northward return back to Canada in summer. Every fall, millions of these butterflies fly west to their wintering grounds in California and Mexico, covering the trees there with their bright shimmering wings. These butterflies congregate into colonies, clustering onto the pine and oyamel trees. In many cases, they are so thick that the trees turn orange in color and branches sag from the weight. It’s a remarkable sight that attracts scores of tourists.
The Monarchs are the only butterfly that migrates both north and south as the birds do regularly, but no individual makes the entire round trip, because the migration period spans the life of three to four generations of the butterfly. Monarch butterflies are also one of the few insects which can cross the Atlantic.
The Monarch migration usually starts around October each year, but can start earlier if the weather turns cold sooner. They travel between 1,200 and 2,800 miles or more from Canada to central Mexican forests where the climate is warm. If the monarch lives in the Eastern states, usually east of the Rocky Mountains, it will migrate to Mexico and hibernate in oyamel fir trees. If the monarch butterfly lives west of the Rocky Mountains, it will hibernate in and around Pacific Grove, California in eucalyptus trees. Monarch butterflies use the very same trees each and every year when they migrate, which seems odd because they aren’t the same butterflies that were there last year.
How the species manages to return to the same overwintering spots over a gap of several generations is still a subject of research. Some believe the flight pattern is inherited. Other researches indicate the butterflies navigate using a combination of the position of the sun in the sky and the earth's magnetic field for orientation.
For years, people puzzled where the millions of Monarchs that spend the summers in Canada disappear to in winter. Then in 1937, Canadian zoologist F. A. Urquhart started tracking the trails of the butterflies by tagging the wings of thousands of individual Monarchs. 38 years later, and with the help of thousands of volunteers across the country, Urquhart located the first known wintering refuge on a mountaintop in Michoacán, Mexico, more than 4,000 kilometers from the starting point of their migration. The area is now a World Heritage Site known as the Monarch Butterfly Biosphere Reserve. There are dozen such sites in Mexico and they are protected as ecological preserves by the Mexican government.
Unfortunately, the Monarchs’ overwintering sites are under threat because of massive deforestation. This year, the number of monarch butterflies that completed the migration to the Mexican forest dropped to its lowest level in at least two decades, due mostly to extreme weather and rapid expansion of farmland. Because the insects cannot be counted, the combined size of the butterfly colonies is used as a proxy in the census. The area of forest occupied by the butterflies, once as high at 50 acres, dwindled to 2.94 acres in the annual census conducted in December 2012.
The Monarchs’ migration is a natural marvel and for Mexico, a huge tourist attraction. It would be a shame to lose it.