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Monitoring Migrating Monarchs in Monterey County

Ventana Wildlife Society has been conducting surveys of overwintering monarch butterflies (from 2001 through 2008) to monitor roosting sites along the California coast in Monterey County and to document monarch butterfly population densities, fluctuations, and movements.  Monarch butterflies are amazing insects with a wonderfully unique life cycle, and by studying their overwintering populations we hope to learn more about why they spend the winter where they do, and what we can do to ensure their return to the central coast year after year.

Survey crew

Roosting MonarchsRoosting Monarchs

Two roosting MonarchsTwo roosting Monarchs
Clusting MonarchsCaptured monarch butterflies

Migration MapMonarch butterfly migration map

Monarch butterfly clusting on Pine trees
Monarch butterfly map

Explore the links below to learn more about Monarch butterflies, our research, and what you can do to help!

Monarch Biology
Learn more about the Monarch life cycle, their amazing migration, and the overwintering sites where they gather.  Check out the Monarch Watch website.

VWS and Monarchs
Find out how we started, how we study the butterflies, and what we’ve found.

Monarch Butterfly Updates
Check this page for updates on the latest Monarch Butterfly numbers, research findings, and media.

Check this page for VWS monarch-related events and workshops.

Monarch Alert
Ventana Wildlife Society works with California Polytechnic State University, San Luis Obispo to operate Monarch Alert. Learn more about Monarch Alert and our partnership here.

Meet project sponsor Helen I. Johnson
A special thanks to Helen Johnson for years of dedication and support.

VWS and Monarchs: Western Monarch Butterfly Population Dynamics, Monterey Co., California

Monarch populations west of the Rockies, migrate to specific overwintering sites along the coast of California.  In California, upwards of 200,000 western monarchs butterflies (Danaus plexippus L.) overwinter annually.  These sites extend from Marin County in the North to San Diego County in the South.  In some winters, Monterey County may accommodate as much as 35% of the western monarch population.  However, overwintering sites of monarch butterflies are identified by the California Department of Fish and Game as vulnerable in the state; at moderate risk of extinction due to a restricted range, relatively few populations (often 80 or fewer), recent and widespread declines, or other factors. 

In 1983, the International Union for Conservation of Nature and Natural Resources created a new category to list the overwintering sites and consequently, the annual migration of monarch butterflies in California and Mexico as threatened (Wells et al. 1983). Monarchs suffer from recurring natural threats to their overwintering sites and human influenced habitat loss and degradation.  Despite the monarchs’ wide public appeal and status as a species of conservation concern, protection of the overwintering sites and understanding of population dynamics in western North America are limited. 

Monarch butterflies require specific habitat and microclimate characters for overwinter survivorship.  Monarchs choose sheltered groves of trees close to the ocean, in areas buffered from freezing winter temperatures and severe storms. Suitable habitat is typically composed of trees in an amphitheater, or U-shaped formation with several rows of trees on the windward side of the grove that allows light to penetrate for warmth.  Groves must also have vertical density, a multi-tiered canopy for adequate protection from wind, cold and storms.  Light should penetrate enough to allow insolation, but not enough to significantly heat the butterflies while in clusters.  Too much heat and butterfly metabolic rate increases necessitating finding nectar and shortening overwintering life span.  Some winter feeding is normal and so groves should be close to water and nectar. 

Historically, butterflies likely utilized native conifer stands of Monterey pine (Pinus radiata), Monterey cypress (Cupressus maculatum) and Coast redwood (Sequoia sempevirens), but in the last century monarchs have been observed roosting in introduced eucalyptus.  Eucalyptus have strong vertical layering, grow quickly and densely, and are not susceptible to disease while Monterey pine is susceptible to pitch canker and is not densely constructed.  Further, extensive land development, logging, and poor land management have reduced the number of native tree stands that support over-wintering monarchs.  Use of isolated stands of non-native eucalyptus trees makes monarchs vulnerable to land management plans that mandate the removal of non-native tree species.  Today, protection and management of monarch overwintering sites usually entails balancing planting eucalyptus coupled with its eradication and removal of diseased pine. 

Given the unique and precarious circumstances of the monarch butterfly’s existing overwintering habitat, it is essential to monitor numbers of monarchs and locations supporting them in order to make scientific management recommendations to sustain future monarch butterfly populations. In winter 2001, Ventana Wildlife Society (VWS) in collaboration with Helen Johnson and California Polytechnic State University at San Luis Obispo embarked on a research initiative to document population dynamics, health and roost site quality of monarch butterflies in Monterey County.  Results of that collaboration identified 9 important autumnal and true overwintering sites in Monterey County.  These include The Monarch Grove Sanctuary, George Washington Park, Point Lobos State Reserve, Palo Colorado Canyon in Big Sur, Andrew Molera State Park, Sycamore Canyon at Pfieffer Beach, a Private Property site in Big Sur, Prewitt Creek and Plaskett Creek in Pacific Valley.  Sites are managed by California Department of Parks and Recreation, the Forest Service, the City of Pacific Grove and private citizens.  VWS has worked with each of these landowners to manage their respective monarch groves by producing reports and making recommendations.

In addition to collecting population data, monarch butterflies are also tagged in order to study inter-site movement and spring dispersal patterns.  Permission was granted by the Pacific Grove City Council for VWS to tag at Monarch Grove Sanctuary.  Two sessions took place at that location, one on November 2, 2006 (1,075 butterflies tagged), and the other on December 19, 2006 (73 butterflies tagged).  Because numbers were dwindling at Monarch Grove Sanctuary, VWS was granted special permission by the Pacific Grove City Council to tag at George Washington Park.  The final tagging session was at that location on February 23 (741 butterflies tagged).  Results from the tagging indicate that monarch butterflies move between sites throughout the winter.  Butterflies tagged at the Monarch Sanctuary were re-sighted at George Washington Park, Point Lobos (11 miles south), Natural Bridges State Park (40 miles north), and a private property site in Big Sur (40 miles south).  These results indicate that protection of multiple overwintering sites is crucial to the survival of monarch butterflies. 

Methodology: Site Evaluation
To determine if monarch butterflies utilize a site for overwintering, VWS biologists will visit a prospective site between sunrise and 0900 to determine habitat suitability, evaluate stand condition, and search for presence of monarch butterflies in clusters.  The entire site survey takes no more than 30 minutes.  If monarchs are found, site monitoring will follow (see below).  If the habitat looks suitable, but monarchs are either not found or not found in clusters, a single follow-up visit will ensue 30 days after the initial survey.

Monarch butterflies Clustering in Eucalyptus tree
Researching Monarch butterflies

Methodology: Monitoring

This project uses protocols established during the past four winter seasons.  Surveys are conducted one day per week from October 1 to the last week of February in the mornings while temperatures are low (below flight threshold, 13°C) and monarch butterflies are still clustered.  Surveys do not take place during heavy precipitation because of poor visibility, but will be made up on the next available “good weather” day.

Data is recorded at each site using a standard data form.  These data include: date, site, observers, pre-count time start and end, count time start and end, presence of nectar and water sources, and observations of tagged or mating monarch butterflies. For every tree that has roosting monarch butterflies, number of butterflies, tree species, tree identification number, and the aspect and height of clusters will be recorded. Furthermore, the number of monarch butterflies on the wing and on the ground is counted and recorded separately. To estimate the number of butterflies in a cluster, we first estimate the number of monarch butterflies in a small area of a cluster, and then extrapolate this count to arrive at a total count for the entire cluster. The average of the total counts of all observers is then recorded. Total butterflies on each tree are calculated by summing the cluster totals. Aggregation aspect, the range of directions that butterflies are roosting relative to the base of the tree, will be recorded. These ranges are converted to presence or absence of butterflies at eight cardinal directions (N, NE, E, SE, S, SW, W, and NW). For each tree that has butterflies, the number of butterflies present will then be evenly distributed throughout the range of directions in order to weight aspects for cluster size. 

Methodology: Tagging

Monarch butterfly with id# tracking and telephone number to call when found
Capturing Monarch butterflies for tagging

Butterflies are captured from overnight roosting clusters during the early morning using a 10 meter multi-section pole with a net attached. Tags used in the study are small round self-adhesive stickers (Watson Label Products, St. Louis, Mo.). Each tag has a preprinted identification number, the words “FREE CALL”, and the Ventana Wildlife Society toll-free number. Tags are gently placed on the underside of the right hindwing of the butterfly. After tag placement, the sex, body condition, and mated status of each individual will be recorded.  Data will be recorded at each site using a standard data form. 

VWS is involved in Monarch Alert, a collaborative research project with California Polytechnic State University at San Luis Obispo to study patterns of fall migration, wintering activity, and spring dispersal of tagged monarch butterflies in western North America.  For more information about Monarch Alert, visit their website at

Methodology: Reporting
Each year, project investigators prepare and submit monitoring reports using data collected from monarch butterfly surveys, tagging, and weather (collected from data-loggers) (Hamilton et al. 2002, Frey et al. 2003, Hamilton et al. 2003).  The monarch project has been featured in over 30 media outlets both locally and nationwide, as well as in several newsletters, including Monterey Peninsula Audubon Society and California Audubon Society. 

Monarch Butterfly Updates and Discussion Board

Check this page for updates on the latest Monarch Butterfly numbers, research findings, and media or link direcly to our western monarchs discussion board.

One of the action items outlined from our meeting last December (Western Monarch Symposium, Pacific Grove Museum of Natural History, Dec 2, 2006) was to create a discussion board on yahoo so that we could freely share information and communicate more regularly about western monarchs. We invite you to join in the discussion.

Group name: western_monarchs  

Group home page:

Group Email Addresses
Post message:
List owner:

Peak number of butterflies

This graph demonstrates the cyclical nature of monarch butterfly overwintering population dynamics.  Abundance of overwintering monarchs seems to be tied with abundance and productivity of milkweed on the summering grounds, which in turn is linked to rainfall and land management.

Monterey County Thanksgiving Butterfly Count 1997-2007

In Monterey County, Pacific Grove houses the highest numbers of butterflies.  In the winter of 2006/7, it held 68% of the monarch butterflies during Thanksgiving week.


PG Sanctuary

GW Park

Point Lobos SR

Andrew Molera SP

Private Property site

Plaskett Creek


























no count






































Monarch butterfly numbers from biweekly counts conducted during the winter of November 2006- February 2007.


Western Monarch Butterfly Symposium held Dec 2-3, 2006, Pacific Grove Museum of Natural History and co-sponsored by Helen Johnson and Ventana Wildlife Society.  The symposium themed “To define working sustainable solutions for the preservation of monarch butterflies and their habitats”featured the following speakers and presentations: Stuart Weiss, Ph.D., Consulting Ecologist, Creekside Center for Earth Observations, Microclimate assessment and canopy structure design for overwintering monarch butterflies; Walter Koenig, Ph.D., UC Berkeley, Hastings Reserve, Spatial synchrony in monarch butterflies; Jessica Griffiths, Ventana Wildlife Society, Monitoring Monarch Butterflies in Monterey County; Mia Monroe, Monarch Campaign Coordinator, The Xerces Society, Citizen Science and Western Monarch Butterfly Conservation; John Dayton, What Makes a California Overwintering Site; Ro Vaccaro, Friends of the Monarchs, Monarch Moments; Bobby Gendron, Owner, Butterfly Encounters, Milkweed Cultivation for California Gardens; and Jan Southworth, Ranger, East Bay Regional Park District, Coyote Hills Regional Park Coyote Hills Butterfly Garden and our Monarchs in the Classroom program for teachers.

Conservation of the Monarch Butterfly — Population Dynamics and Migration held Thursday and Friday, Dec. 8-9, 2005 at California Polytechnic University at San Luis Obispo.  Dr. Dennis Frey and Shawna Stevens, co-leaders of Project Monarch Alert, headed up the conference and sponsored by Helen Johnson.  Featured speakers included Sonia Altizer, University of Georgia Ecology Institute; Karen Oberhauser, University of Minnesota Department of Fisheries, Wildlife and Conservation Biology; and Chip Taylor, Kansas University Entomology Department, among others. Altizer is an international authority on monarch ecology and evolution, as well as the effects of parasites on the insect. Oberhauser directs the Monarch Larval Monitoring Project that studies monarchs nationwide at their breeding grounds. Taylor is director of Monarch Watch, an educational outreach program that engages the public in large-scale research projects.

Meet Helen Johnson

Helen Johnson
Helen Johnson

by: Jim Davis, former Executive Director of Ventana Wildlife Society
Helen Johnson is an extraordinary person of great commitment and energy. She has been called a citizen scientist.  As a citizen she feels that this is a world that needs help and she is there to see that important things get done to help it. One visit to her home and you know that this is a person who lives a humble life and has committed herself to the improving the natural world. Birds, butterflies and native plants to support them abound in her yard and home. As a scientist Helen has funded an effort with VWS to study the monarch butterfly. She engages as many people as she can to get as much done as possible, and has provided staff with training and equipment for the project. In addition to supporting, Helen provides follow-up on what is done and how it is done. She is always one or two steps ahead of the rest of the world. We need more people like Helen Johnson; our world is a better place because of her. Thanks Helen, it is nice to know someone like you. –Jim