Volunteers with the Western Monarch Thanksgiving Count have been monitoring the numbers of monarch butterflies overwintering in California every fall for the last twenty years. To better understand overwintering monarch clusters’ persistence during the overwintering season, the Xerces Society and Western Monarch Thanksgiving Count volunteers launched a New Year’s Count in January 2017.
The first New Year’s Count took place in January 2017 at 44 overwintering sites throughout the monarch’s core overwintering range including key sites such as the Monarch Grove Sanctuary in Pacific Grove, Lighthouse Field State Beach in Santa Cruz, and the Monarch Butterfly Grove in Pismo Beach. This second standardized count period provides a comparison to the count done each fall around Thanksgiving and can help begin to answer questions about winter mortality, differences among sites, and site management.
In the period from Thanksgiving Count 2016 to New Year’s Count 2017, monarch numbers at monitored sites decreased by an average of 43%. Of the 44 sites included in the New Year’s Count, monarch numbers decreased at 33 sites, stayed the same at 2 sites (both had no monarchs), and increased at 9 sites. This decrease in numbers may be due to a combination of mortality (due to storms, predators, or other causes) and movement of monarchs between sites. California’s winter of 2016-2017 had higher than average rainfall, which may have contributed to monarch mortality this year, but without multiple years of data, we do not know if 43% is typical. To that end, the Xerces Society is looking for additional volunteers to expand the New Year’s Count in January 2018 to more overwintering sites in order to better understand the extent of overwintering mortality, as well as gain additional information about population trends and site-level differences.
You can view all of the data from the Thanksgiving Count and New Year’s Count on the Western Monarch Thanksgiving Count website here.
Thank you to all of the regional coordinators and volunteers who make the Thanksgiving and New Year’s Counts possible! We recently celebrated the 20th anniversary of the Thanksgiving Count and look forward to many more years of counting.