Monarch Butterfly Biosphere Reserve - Monarch Migration

Monarch Migration

The monarch butterfly is noted for their lengthy annual migration. It is the only butterfly species known to make annual north-south migrations like many bird species do. Monarchs in North America roughly divided into two migrations. Those in the west tend to migrate toward California for the winter and those east of the Rocky Mountains migrate south. However, no single individual makes the entire round trip, as butterfly lifespans vary from just two months to about seven months for those who hibernate It is not clear how these monarchs know how to return to the same wintering sites as their ancestors, but flight patterns seem to be inherited.

Wintering sites for these eastern butterflies are thought to be as far north as Texas and Oklahoma . However, most travel south into Mexico, crossing the border south from Texas and then following the Sierra Madre Oriental mountains. After thirty years of studying the migrations of eastern monarchs, zoologist Fred Urquhart discovered in 1975 the site where the butterflies migrate to. They spend the winter in the forests of pine and oyamel fir in central Mexico, along the border of the states of Michoacán and Mexico. For those who come from the U.S./Canadian border area, this represents a voyage of about 4,000 km (2,485 milles). This southward migration concentrates a large number of insects from a widely dispersed area in the north to a small area in the south. This kind of migration is unique in the world. These butterflies spend about five months in this area of Mexico, arriving in October and leaving in March.

It is estimated that anywhere from 60 million to a billion butterflies arrive to the central Mexican highlands every winter, mostly clustering in the small area along the border of the states of Michoacán and Mexico. 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. When these butterflies take flight for mating purposes, they can fill the sky and appear like clouds and the beating of their collective wings has been compared to light rain. Although these forests are located in central Mexico, their altitude makes the area susceptible to low and freezing temperatures at night. If the weather gets too cold, the butterflies will die. Low temperatures at night each year kill a certain percentage every year, usually the weakest. However, unusually cold winters, such as the one that occurred in 1996/1997, can kill off so many that their bodies cover the ground. However, not all butterflies that fall to the ground overnight are dead. Some will reanimate when the sun rises in the morning. One reason the butterflies congregate so thickly on trees is to conserve heat. At dawn, the butterflies will open their wings to catch the sun’s warming rays. When the temperatures get warm enough in the daytime, the butterflies will rise up into the air en masse for mating.