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2013-14 California Overwintering Population Numbers Released

Feb 03, 2014


  • MJV Partnership News
  • Population Trends

The Xerces Society for Invertebrate Conservation released data illustrating the 2013-14 monarch overwintering population in California. In order to determine the size of the western overwintering population, volunteers participate in the annaul Western Monarch Thanksgiving Count, and this year visited 162 sites in coastal California, tallying 211,275 monarchs in total. While this number shows a slight improvement from last year's population estimate, the number of butterflies at overwintering sites in California is still well below the peak numbers from the 1990s. 

The data from the Western Monarch Thanksgiving Count can be viewed in more detail at

The overwintering phenomenon in California is similar to that in Mexico, but there are differences that make it more difficult to track the size of the western population. There are many smaller sites in California which monarchs occupy during the winter, making it more difficult to estimate the population using the area of land occupied by these monarchs. Now in its 17th year, the Western Monarch Thanksgiving Count is a remarkable citizen science effort to engage volunteers in estimating the western overwintering population. Over a three week period spanning the Thanksgiving holiday, dozens of volunteers visit scores of overwintering sites to count the number of monarchs. Overwintering monarchs along the California coastline have shown considerable declines since the count began. In 1997, the first year of the count, there were over 1.2 million monarchs at 101 sites. In 2013, just 211,275 were counted at 162 sites.

Loss and degradation of both breeding and overwintering habitat, pesticide use, and drought—exacerbated by climate change—may all be contributing to this decline. Conservation scientists are concerned that the continuing drought in California and throughout the west may lead to even lower numbers in years to come.

Volunteer efforts such as the Western Monarch Thanksgiving Count are critically important to understanding the status of western monarch butterflies, and the 2013 overwintering season witnessed the most comprehensive site coverage to date, with 162 sites visited from Sonoma to San Diego Counties. Groups such as the Xerces Society, Monarch Alert, the Pacific Grove Museum of Natural History, and the Monarch Program are working to increase volunteer participation by providing outreach opportunities and engaging local citizens and students.

In addition to conserving and monitoring overwintering habitat for monarchs in California, we need to also create and restore breeding habitat (areas with milkweed) for monarch reproduction. In the spring as the overwintering clusters begin to disperse, monarchs move inland in search of milkweed on which to lay their eggs. In partnership with the Monarch Joint Venture, the Xerces Society is working to identify potentially important breeding areas in the western U.S. and to inform and promote better land management practices to encourage milkweed to grow in the west (see In addition to identifying these breeding habitats, we strongly encourage you to get involved in monitoring a site or sites for monarch eggs and larvae through the Monarch Larva Monitoring Project. There are instructional videos for how to get involved in this citizen science effort on the project website:

Spring is approaching - have you thought about your monarch conservation plans for the upcoming breeding season? Plant native milkweeds in your garden or another natural area, promote conservation by displaying signs at your site or giving educational talks, and support monarch conservation organizations like the Monarch Joint Venture so that we may continue to support our partners engaging in monarch conservation and education throughout the U.S.!

For More Information

For information about the Western Monarch Thanksgiving Count and how to get involved, visit

Read more about Xerces’ Monarch Conservation Campaign, including efforts to conserve overwintering sites in California and restore breeding habitat in key regions of the United States at