Monarch Life Span and Migration
Spring and Early Summer Generations
Late Summer Generation
Generally, those butterflies east of the Rocky Mountains migrate to the east-west trending transvolcanic mountain belt near Mexico City, while those west of the Rockies travel to overwintering sites along the coast of central and southern California. Both populations arrive at the overwintering sites around the end of October.
Overwintering sites must provide several conditions such as climate and trees. The butterflies need a cool place, but not freezing, with some moisturelike that derived from fog. Trees are an important element of the overwintering sites. The same trees are often used year after year. In California eucalyptus trees, Monterey pines, and Monterey cypress trees provide roosting sites, while Oyamel Fir trees are the choice in central Mexico. Truly an impressive sight, these trees are covered with clusters of butterflies clinging to the trunks and branches. These sites have become popular wintertime tourist attractions.
In the spring, frequently around Valentine's Day, the migrant butterflies will mate. The males die soon after mating and the females begin their flight north looking for milkweed on which to lay their eggs (possibly 300-400) after which they also die. The lifespan of these migrating butterflies is six or nine months compared with four to six weeks for the spring and early summer generations. It will be the great-great-grandchildren of this generation that make the next migration.
The entire migration phenomenon is truly amazing. These relatively small creatures fly great distances to overwintering sights that they themselves have never visited. How do we know that they do this? How do the butterflies know where to go?
Through the efforts of various groupsincluding school children and many citizen scientiststagging has helped to solve the mystery of where the butterflies go and how far they travel. Small, numbered tags are placed on the butterfly's wing. Useful information such as the tag number, and where and when the butterfly was released, is recorded and sent to a central location where it can be matched with recovered tags.
While tagging has helped to answer the question of where the Monarchs go, the question of how they find their way is more of a mystery. Does it have to do with the position of the sun? Do they use some form of internal compass to guide their way? No one knows for sure.