Having survived winter, loaded up on nectar, and mated, 271,924 monarchs are leaving California now and flying to their summer ranges in the western portion of the US and Canada. Mexico’s overwintering population data hasn’t been officially released yet because those monarchs stay until March before migrating to their summer homes east of the Rockies*. Both overwintering regions offer similar habitat features to support the long-lived monarch super-generation that migrates south at the end of summer, waits until spring, and then begins the migration north. And because overwintering is risky for the monarchs who must survive and mate before migrating, census information can’t be considered final until spring migration. In some years unusual cold (or heat), absence of moisture, or too much rain has killed monarchs and severely reduced the populations before spring migration began.
The Xerces Society recently released population data on the 2015-16 monarchs overwintering along California’s coast. Although this year’s population size is greater than last year’s and more sites were occupied, 271,924 is only a fraction of the 1.2 million recorded in the late 1990s and is 39 percent lower than the longterm average population size. Historically, over 450 sites have been documented since the 1980s.
There are positive indications from this year’s count results. Fifteen sites that have been continuously monitored had the highest numbers of butterflies in a decade. Several sites that had not seen monarchs for years were occupied, and there were a number of sites, such as the Berkeley Aquatic Park, that hosted overwintering monarchs for the first time. In Marin County in the northern extent of the overwintering range, 11 sites had increased numbers and two new sites each supported more than 8,000 butterflies.
Less positive is that in southern California, the majority of the sites surveyed had fewer monarchs than last year.
Overwintering populations in California 1997-present season (2015)
Monarchs who summer west of the Rockies in the US and Canada migrate to overwinter on the California coast. Butterflies who summer east of the Rockies overwinter in the State of Michoacán, Mexico’s high elevation mountains (9 -10,000 feet) with oyamel fir (Abies religiosa) and white pine forests (Pinus pseudostrobus).* Both overwintering habitats are buffered from extremes of heat and cold, and have adequate moisture from fog to prevent the butterflies from drying out. (There are some exceptions to this strict division of migration patterns according to monitoring by the Southwest Monarch Study. Occasionally a western monarch will go south to Mexico.)
In California, the winter climate, coastal belt fog, and specific trees provide everything monarchs need to survive during the winter: protection from temperature extremes, moisture to prevent dessication, and a source of nectar from early spring flowering trees. In Mexico, the high elevation volcanic slopes of Michoacán’s oyamel fir and white pine forests offer similar habitat features. Moisture from fog gathers on these slopes and the trees provide protection and nectar.
Their winter habitats are specialized and vulnerable to climate change impacts that cause unusual weather or change long-standing conditions. There are reasons the butterflies go to only these two regions and if these alter, such as less fog, monarchs suffer higher losses during winter.
In 2002, an usually wet and cold weather system caused catastrophic losses of Mexico’s overwintering monarch populations. This highlighted the importance of the overwintering populations to spring migration and summer populations. Monarchs can produce a huge amount of eggs (one female can lay 400 eggs in their lifetime) but without protection of both summer and winter habitats and the migration corridor that connects them, monarch populations are at risk.
Historically, California’s native Monterey pines (Pinus radiata) and incense cedar (Calocedrus decurrens), and local species of cypress (Cupressus spp.) served as host trees. But Monterey pines and their specialized coastal terrace habitat have been nearly wiped out by development (ocean view homes won). The same people who began eliminating the pines introduced eucalyptus from Australia, thinking it grew fast, resprouted from cut stumps, and was going to be profitable timber. But eucalyptus (primarily Eucalyptus globulus) doesn’t mill into useable timber so the trees went unharvested and flourished. Fortunately, the trees took over the pines’ coastal habitat, bloom in early spring, and now supplement the protection and nectar provided by the remaining trees.
Monarchs cluster together in huge masses hanging from the trees as a means of conserving their body temperature and moisture. Dr. Fred Urquhart, in his 1976 National Geographic article announcing the discovery of the Mexican overwintering habitat, describes how the biology of monarchs and their overwintering habitat climates interact. Similar habitat characteristics occur in California’s overwintering sites under a different set of conditions.
Butterflies are poikilotherms, that is, creatures that adjust their body temperatures to the ambient air. At this 9,000-foot elevation [in Mexico], winter temperatures hover from just below freezing to just above. Ideal for monarchs! Inactivated by the chill, they burn up almost none of the reserve fat they’ll need on their northward flight.
Although individual butterflies weigh less than a gram, tens of thousands of butterflies gather tightly into clusters and need strong branches that can support their weight. Mexico’s oyamel fir and white pine and California’s Monterey pine, eucalyptus, cypress, and incense cedar have sturdy branches that can hold the weight. The trees have a dense crown and together form a solid canopy that moderate temperature swings and strong rain. On sunny days, the clustered monarchs separate and bask in the sun, returning to cluster when night approaches.
In spring, warming temperatures and increasing day lengths signal monarchs that it’s time to leave their overwintering areas and move north. The monarchs become more active, and mating begins. The trees that sheltered them all winter in Mexico or California produce flowers. Nectar from these flowers fuels the monarchs for their trip north, helping them survive long enough to find the first milkweed host plants.
While only one generation migrated south, several generations are needed to complete the northward migration to their summer ranges. The first step of the new season for western monarchs has begun and now it’s up to the butterflies finding suitable milkweed host plants for eggs and larvae, nectar plants for adults' food, and avoiding pesticides and other dangers.
* SEE THESE PREVIOUS STORIES ABOUT MONARCHS FOR MORE INFORMATION AND PHOTOS.
Monarchs Help Solve Their Own Mystery (but local people always knew the answers) tells the story of the 1976 discovery of the monarchs’ unknown overwintering locations in Mexico that now are a UNESCO BioSphere Reserve.
Monarch Magic at El Rosario Sanctuary presents early census data for the largest overwintering site in Mexico and describes how the local residents are changing their way of life to protect the monarchs and their forest, and to promote ecotourism instead of logging and clearing forests for agriculture.
Sleeping milkweeds wait for spring monarchs (plus planting guide for your area) is what it says: basic information and links to resources to guide local planting of milkweeds that help monarchs (and why the wrong milkweed species can harm them).