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Counting Western Monarchs

Jan 13, 2016


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Written by Candy Sarikonda, Monarch Watch Conservation Specialist and Wild Ones Ohio Member.

The fall monarch butterfly migration is a spectacular phenomenon.  Since monarchs cannot survive freezing cold winter temperatures in the north, they must migrate to more suitable habitat to spend the winter.  Most monarchs east of the Rocky Mountains migrate to overwintering sanctuaries in central Mexico.  However, the majority of monarchs west of the Rockies choose to migrate to one of over 200 different overwintering sites along the California coastline.  Thousands of western monarchs spend the winter at these sites along the coast, usually arriving in October and staying until mid-February.  Many sites have been observed by monarch scientists, enthusiasts and volunteers for years, and trained volunteers conduct monarch counts at numerous sites annually during a 3-week period around Thanksgiving Day.  Some overwintering sites are monitored more frequently throughout the winter, and I was given the opportunity to participate in the bi-weekly monarch population count at the Pacific Grove Monarch Butterfly Sanctuary (PGMBS) in Pacific Grove, CA.

The PGMBS has long been a preferred site of overwintering monarchs.  Western monarch population counts have been conducted and documented annually at PGMBS and other California sites since 1997, via an effort coordinated by the Xerces Society.  The population counts are invaluable for understanding the overall western monarch population, providing researchers with a picture of the status of the population and its conservation needs.  Bi-weekly overwintering population counts are done at PGMBS, allowing researchers to note fluctuations in the grove's population over the season.  In addition, tagging was conducted by the Pacific Grove Museum of Natural History at the sanctuary in 2014, enabling researchers to track monarch movements within and between overwintering sites.  Parasitism studies were also conducted, allowing researchers to determine the prevalence of the Oe parasite in monarchs at the site.  The tagging and parasitism data were shared with researchers at Cal Poly San Luis Obispo’s Monarch Alert program and the University of Georgia’s Monarch Health lab to further understand western monarchs’ overwintering behavior and parasite prevalence.  A great deal of valuable information about western monarchs has been gleaned from the efforts of volunteers and scientists at the PGMBS site, and I was honored to have the opportunity to participate in the PGMBS monarch count.  

I joined Allison Watson, education programs manager for the Pacific Grove Museum of Natural History, to conduct a count of the monarch butterflies spending the winter at PGMBS.  The grove is a forest of native Monterey cypress and Monterey pine, as well as non-native eucalyptus trees.  In the winter, monarchs gather together in groups, or clusters, to roost in the tree branches during the cold nights.  This overwintering season monarchs arrived to PGMBS early, arriving in early October and quickly increasing in number.  By Thanksgiving, monarchs at the grove numbered 11,533.  

Ms. Watson and I met in the grove at sunrise the morning of December 30, 2015.  Early morning is the best time to observe inactive, roosting monarchs.  That morning it was 42F, clear skies with calm winds, and we found the monarchs roosting primarily in a single Monterey cypress tree overhanging the driveway near the grove entrance.  Ms. Watson explained that when monarchs first arrive to the grove at the beginning of the overwintering season, they typically cluster in the line of eucalyptus trees near the grove entrance.  But when air temperatures begin to decrease as the overwintering season progresses, the monarchs move to trees near the center of the grove where there is more protection from wind and inclement weather.  Each tree has a small identification tag placed on it, allowing volunteers to document exactly which trees the monarchs are using.  Ms. Watson explained, "The grove is experiencing some decline.  Drought and disease are affecting some of the trees.  We lost two trees to pitch canker this year."  By collecting data on the monarchs' tree usage along with concurrent temperature, wind and weather conditions, restorationists can determine how best to restore the grove to meet the needs of the overwintering monarchs.

Ms. Watson and I observed each tree that hosted roosting monarchs. The cold air temperatures led the monarchs to cluster in tight groups, giving the appearance of clusters of dead leaves hanging from the tree branches.  Ms. Watson examined the clusters with her binoculars, counting the monarchs on each tree branch.  While eastern monarch population counts are done by calculating the area of forest occupied by monarchs, the western population is small enough that monarchs can be counted individually.  Some tree branches contained a handful of monarchs, so each individual monarch was counted.  Other branches contained hundreds of monarchs, and Ms. Watson used a method of estimation to count them.  She explained, "I begin at the tip of the branch, and count ten monarchs grouped together in the cluster.  Based on the amount of space those ten monarchs occupy on the branch, I then count by groups of ten up the branch, until I count all of the monarchs on the branch." One cypress branch, hanging almost straight down due to the weight of the monarchs, held 1000 monarchs!

As we moved further into the center of the grove, Ms. Watson noted a very tall pine tree which held a few small clusters.  "I don't normally see monarchs way up there, so this has been interesting to see," she explained.  She also pointed to a small number of deceased monarchs on the forest floor, with their abdomens missing.  While mice do prey on monarchs on the ground, volunteers have witnessed a squirrel removing monarchs from the clusters.   Reports indicate that the squirrel approaches a cluster, removes a few monarchs, then returns to a protected spot near the forest floor where it consumes the monarch abdomens.  "We were really surprised to see this.  We think it is only one squirrel doing it, but it is really unusual."

As we walked in the cold morning air - hands chilled while holding binoculars, clipboard and camera - I noticed the strain on my neck.  All that looking up in the trees for long periods was not easy!  Ms. Watson laughed, "Yes, it is a bit of a strain.  I definitely feel it in my shoulders but I am getting used to it.  I might treat myself to a shoulder massage at the end of the season." I nodded in absolute agreement.  After an hour of counting, the count was complete.  Ms. Watson tallied the results.  The final count was 10,236 monarchs at the grove!   

We discussed the results of the count.  Monarchs numbered 24,000 at PGMBS during the previous overwintering season, indicating this season's count was a drop from last season.  But notably, preliminary results from the Western Monarch Thanksgiving Count suggest a slight increase from last season in the overall western monarch population.  Reports are currently being gathered from more than 85 volunteers who surveyed more than 130 sites.  According to Sarina Jepsen of the Xerces Society, "...the surveys so far indicate that sites north of Santa Cruz are hosting more butterflies than previous years; whereas sites in Monterey, San Luis Obispo, and Santa Barbara Counties are reporting fewer numbers of butterflies on average. Several new sites have been reported, including some from Marin County with up to 10,000 monarchs. The data is not yet available for Santa Cruz County and many sites in southern California." Ms. Watson expressed excitement over the discovery of new overwintering sites, and how these new sites might fit into the picture of the overall western monarch population.

My visits to PGMBS are always an awe-inspiring experience.  But having the opportunity to participate in a monarch count was a unique learning opportunity that I will treasure for years to come.  If you are interested in participating in a monarch count at a site near you, please consider volunteering by contacting the Xerces Society.  Training with experienced counters is recommended for new citizen scientists.  Volunteers are especially needed to commit to assessing a site year after year. Learn more here:  Get started today!


Figure 1: Allison Watson recording count data


Figure 2: Cluster of Monarchs on Cypress at Pacific Grove Monarch Butterfly Sanctuary



December 2015-January 2016 photos from PGMBS

Article by Sarina Jepsen of Xerces Society on the current status of the western monarch population

Western Monarch Count Resource Center website, including preliminary count data by site

MJV news article explaining the behavior of the eastern overwintering population and count methodology

Journey North slideshow showing how the eastern monarch population counts are done

The content in this article does not necessarily reflect the positions of all Monarch Joint Venture partners. All photos were taken by Candy Sarikonda during this December 2015 visit to PGMBS.