It is thought that monarchs were originally tropical butterflies that underwent range expansion. Scientists are not sure how long the monarch's spectacular annual migration to Mexico has been occurring; it may be as old as 10,000 years (when the glaciers last retreated from North America) or as young as a few centuries. The earliest reports of overwintering clusters of monarchs in the United States date back only to the 1860’s in California. 

The sites the monarchs use during the winter have particular characteristics that enable their survival. These characteristics are important because they provide the monarch with the right overwintering conditions. Trees on which to cluster are one of the most important elements of the sites. The climate and the entire surrounding area are also important. Nearby trees, streams, underbrush, and fog or clouds all form an intricate natural ecosystem comprising the monarchs’ winter habitat. Monarchs need a cool place to roost so that they don’t use up their energy reserves as quickly. They also need to be protected from snow and winds. The surrounding trees serve as a buffer to the winds and snow.

Although monarchs are found in many areas of the world, the most spectacular migration occurs in North America.

Western North America

Monarchs that spend the summer breeding season in western North America (including states west of the Rocky Mountains: Washington, Oregon, California, Idaho, and Montana) are thought to migrate to the southern coast of California. Here, they roost in eucalyptus trees (as shown in this image), Monterey pines, and Monterey cypresses that are located in bays sheltered from wind or farther inland where they are protected from storms. There are at least 25 predictable overwintering aggregations in California in addition to many temporary clusters. Scientists estimate that the California monarchs make up about 5% of the overall worldwide monarch population.

It has been proposed that this western North American population is not truly migratory but rather undergoes an annual range expansion and contraction. That is, these monarchs may be year-round residents of California whose offspring are able to spread to surrounding states during the mild summer weather but are forced to return to California or perish when the inhospitable northern winters return. This issue is still being debated and offers great potential (and substantial challenges!) for study by west coast residents.

In Fall (left), monarchs east of the Rockies funnel through Texas to Mexico, while monarchs west of the Rockies migrate to coastal California.  In Spring (right), monarchs recolonize the eastern U.S. and Canada in successive generations.  Less is known about how they recolonize the western states.

Eastern and Central North America

Monarchs that spend the summer breeding season in eastern North America (including states and provinces east of the Rocky Mountains: central and eastern Canada, midwestern and eastern United States) migrate to the Transvolcanic mountains of central Mexico. Many millions of monarchs from these regions fly south to Mexico each fall. Their flight pattern is shaped like a cone as they come together and pass over the state of Texas on their way south. In massive butterfly clouds, they sweep up into the mountain ranges of central Mexico. In 1975 the scientific community finally tracked down the wintering sites of the monarchs in Mexico. Until then, the monarch butterflies’ winter hideouts had been a secret known only to local villagers and landowners.

In Mexico, monarchs roost in Oyamel fir forests, which occur in a very small area of mountain tops in central Mexico. Overwintering sites are about 3000 meters (almost 2 miles) above sea level, and are on steep, southwest-facing slopes. Because monarchs need water for moisture, the fog and clouds in this mountainous region provide another important element for the winter survival of the monarchs. The butterflies choose spots that are close to, but not quite, freezing. They cluster together, covering whole tree trunks and branches, and cling to fir and pine needles. The tall trees make a thick canopy over their heads. Protective trees and bushes soften the wind and shield the butterflies from the occasional snow, rain, or hail. Each of the above elements is important to the butterflies, making up the monarch habitat – trees in which to roost, other trees and shrubs to protect them, the cool air, and the presence of water.

On to:  How do monarchs find the overwintering sites?