The Migration to the California Coast

In 1922, Oliver Jenkins, a professor from Stanford University, wrote "... once, while strolling along a path in the pine woods near Pacific Grove, on Monterey Bay, California, I saw a pine tree gay with bright colors. It looked as if it had burst into the bloom of a thousand bright flowers. I was filled with astonishment at the sight and quickly went nearer to find out what wonderful thing had happened to the sober-colored pine tree to paint it up so gorgeously. On coming near, the mystery was solved. There were hanging from the branches and needles thousands of large, brightly colored butterflies. The mystery of the color of the pine tree was explained, but there was another mystery. That was, where did this great company of brilliant creatures come from, and why did they gather together here in such vast numbers?"

What Oliver had experienced was the western monarch migration and today we know a little more about it. For example, we know that monarchs from west of the Rockies overwinter in 300 or so locations scattered along the California coast from just north of San Francisco to San Diego. It’s into one of these overwintering sites that Oliver had stumbled. As in Mexico, the monarchs cluster in stands of trees, but here they are eucalyptus, Monterey pines and Monterey cypresses. Although hundreds of sites have been discovered along 600 miles (1000 kilometers) of coastline, many hold just tens or hundreds of monarchs. The most populated sites are in the middle of the range where tens, and sometimes hundreds of thousands overwinter at some sites. Some of the more visited sites include Pacific Grove, Natural Bridges, Pismo Beach and Ellwood Main just north of Santa Barbara in Goleta.

Although much smaller than the eastern migration, the western migration shares many things in common with it. As in the east, the western migration begins in the fall but involves monarchs scattered throughout the western states and southern Canada. Like their eastern monarchs, they head south or southwest, but here that bearing takes them towards the California Pacific Coast. As they arrive at the coast, they are first attracted to sites with nearby drinking water and sources of nectar. In these places they begin forming clusters in September and October and the population of these clusters increases as the days go by. These autumnal sites last anywhere from a few days to a few months, although in mild winters they may last the entire season. During normal or harsh winters, the monarchs leave their autumnal sites for permanent sites in November and December. Sites are typically located in sheltered areas protected from strong winds. Like their eastern cousins, while overwintering the monarchs don’t mate and are semi-dormant to preserve their lipid (fat) reserves for spring mating and the flight back towards their summer breeding ranges.

The best time to see the monarchs is in December and January when the permanent colonies are fully formed. By late February and early March the clusters begin to break up and the monarchs mate and start to expand their spring and summer range throughout California and the adjacent states.

One interesting aspect of the California migration relates to the trees they roost in. While people are fighting to protect the monarch’s oyamel forest habitat in Mexico, in California the monarchs must have changed the trees they cluster on in the not very distant past. In the north they roost on Monterey pine and cypress trees, but in the south they mainly use eucalyptus trees. Eucalyptus trees were first introduced from Australia in the 1850’s in the hope they would become a source for lumber. That didn’t pan out but monarchs started to use groves of these non-native species for their overwintering sites. What kind of trees these particular colonies used prior to the 1850s is unknown, but they couldn’t have been eucalyptus.