Each fall, the monarch butterflies in the eastern United States undertake an epic migration from their summer breeding grounds as far north as Canada to central Mexico, where they cluster by the millions atop just 10 to 12 volcanic summits. In their winter colonies, the monarchs cling like ornaments to the trunks, branches, and needles of the oyamel fir trees, which only grow on the high slopes of certain mountains in central Mexico. The eastern population of monarchs heads to the Transvolcanic Mountains, which run west to east across the country around the latitude of Mexico City.
In 2005, the mass arrival of the monarchs at the wintering grounds occurred over the first weekend of November. According to reports from observers at the monarch sanctuaries, the sky was thick with monarchs on Saturday, November 5, the day this image of the region was captured by the Moderate Resolution Imaging Spectroradiometer (MODIS) on NASA’s Terra satellite. For the butterflies east of the Rocky Mountains, the migration path funnels them southward toward Texas, where they cross into Mexico in the states of Coahuila and Nuevo León, flying along the Eastern Sierra Madre.
Among the mysteries of monarch migration is how the butterflies know when to stop following the southeastward-trending Sierra Madre and how they find their way from the Sierra Madre (which they abandon in the region just to the south of the mountains’ label in this image) to the individual summits where they will spend the winter. The area between the Sierra Madre and the winter colony sites in the Transvolcanic Mountains is called the Sierra Gorda, and it is crisscrossed by rivers and valleys that run in many directions. The region appears to lack any distinctive north-south ridge lines that would serve as physical cues for the butterflies.
The close-up image (bottom) shows the winter colony habitat in greater detail. The patchwork of deep green reveals the forested areas where the butterflies congregate. Some of the largest colonies are located to the northeast of the town of Zitácuaro. Like forests across the globe, the oyamel forests that harbor the monarch colonies are pressured by human activity. Logging and land clearing in the region threaten the forests, but national and international efforts are underway to promote sustainable development and ecotourism that will financially support the people who live in the area while preserving this critical habitat for the monarch.
For information on the monarch butterfly migration, please visit the Website of the Journey North project, an educational and “citizen science” activity in which students learn about wildlife migration and seasonal change through the global sharing of their own field observations of migrating wildlife, including the monarch butterfly.