Two main populations of Monarchs exist in North America: one population is east of the Rocky Mountains, and the other is west of the Rocky Mountains. (New evidence, however, suggests that they may intermix.)

The western population of Monarchs makes a migration similar to its eastern counterpart, but winters instead at more than 200 coastal sites along the California coast, from north of San Francisco to the Mexican border. Tens of thousands of individuals may be present at an individual Monarch grove.

The Monarchs that migrate out from these overwintering sites in the spring move up into California's Central Valley and Sierra Nevada foothills, as well as north to Oregon and Washington. By the end of the summer, after three generations have passed, some offspring end up as far north as British Columbia. Although only one to two million Monarchs migrate back to California's coastal groves in the fall (as opposed to 100 million to Mexico), the western Monarchs are still important.

To compare the western migration with that in the east, please click here.

For more information on the basic biology of the Monarch butterfly, please click here.