Monarchs: The Western Population

By now, most people are familiar with the monarch butterfly's amazing migration from as far north as Canada to the mountains of Central Mexico each winter, documented in MOSI's current IMAX film, Flight of the Butterflies. Not all monarch butterflies follow this migration pattern, though. Florida's own monarch population, as we noted in a recent post, does not migrate, and resident populations can also be found in Hawaii and Bermuda. And monarch butterflies west of the Rocky Mountains in the U.S. head in an entirely different direction each fall - they seek winter shelter along the coast of California.

Image via Journey North

The Rocky Mountains are a formidable division between east and west in the United States. They contain the Continental Divide, which separates our rivers - those east of the Continental Divide all eventually flow into the Atlantic, and those west flow into the Pacific. They divide animal and bird species too; only the most intrepid creatures will venture through or over these soaring peaks - they average 7000 ft, and top out at Colorado's Mt Elbert at 14,400 ft. Many species have instead developed separate east and west populations, even going so far as to split into separate species altogether - like Eastern and Western Bluebirds, or Eastern and Western Tiger Swallowtail butterflies.

The monarch butterflies living east and west of the Rockies are the same species; no research has found any genetic differences in the populations. They have similar behavior patterns as well; when fall arrives, the final generation of monarchs hatching west of the Rockies is similar to the "supergeneration" that hatches in the east. They are in a state of reproductive diapause, with no interest in mating or laying eggs. Instead, these butterflies head west to their own wintering grounds, along the California coast. There, they gather in eucalyptus, Monterey pine, and Monterey cypress trees and rest for the winter, stirring only on very warm days for a sip of water from nearby streams.

Though the western monarchs make a much shorter journey (usually only a few hundred miles, instead of a few thousand), they encounter pretty much the same environmental conditions when they arrive in their California wintering grounds. The temperatures are cool with high humidity; think foggy San Francisco instead of sunny Los Angeles. Many of these wintering grounds are very easily accessible to the public, and some communities have set aside specific areas for tourists to visit these wintering colonies. (Click here for a brochure from Monarch Watch listing 25 popular California wintering sites.) Biologists are worried about California's wintering populations, though - numbers have plummeted by as much as 90% in the last 15 years. Many groups are actively working to conserve and protect these wintering grounds and the monarchs who spend time there.

If you want to visit the monarch wintering sites in California, plan to go between November and January. Changing climates and differing seasonal patterns mean that monarchs sometimes arrive late or leave early (most are reported to have left the wintering grounds by mid-January this past winter), so it's best to look for them during the peak wintering time.