Where Do the Monarchs Go?
Knowing When To Leave
Where Do They Go?
Finding Overwintering Sites
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
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
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
In Fall (top), monarchs east of the Rockies funnel through Texas to Mexico, while
monarchs west of the Rockies migrate to coastal California. In Spring (bottom),
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
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.
Continue to: How do monarchs find the overwintering sites?