Monarch Overwintering Population in Mexico

Each fall, monarch butterflies across eastern North America travel thousands of kilometers to spend the winter in the mountaintop forests of central Mexico. This migration is the most spectacular long-distance, two-way migration carried out by an insect. Every winter, since 1993, scientists in Mexico estimate the size of the population of monarchs overwintering there. The graph below (from this 2023 study - in spanish) shows the most recent population estimate in the context of three decades of data.

Total area of overwintering monarch butterflies in Mexico (click on image to enlarge)
Total area of overwintering monarch butterflies in Mexico (click on image to enlarge)

As these data reveal, the abundance of monarchs shows a discouraging downward trend over the past 30 years. Insect populations are notorious for their annual fluctuations, but this most recent (winter 2023-2024) estimate is concerning. It is the second lowest ever reported; only the winter of 2013-2014 was lower (0.67 ha). Read more here.

Determining Population Size

After the monarchs have settled into their overwintering roosts, the area of forest occupied by monarchs is measured, and this measurement is used to estimate the size of the eastern monarch population. It is a complex process that starts with understanding overwintering roost dynamics and then involves locating, mapping and measuring occupied trees.

Measurement of overwintering monarchs in Mexico began in 1993, using the method established by Dr. William Calvert. Briefly, it consists of measuring the geographic area of forest occupied by the overwintering butterflies during the second half of December when colonies are fully formed.

Aerial view of forest with overwintering monarchs
Aerial view of forest with overwintering monarchs, Photo by Lincoln Brower

The area is estimated first by measuring the location and perimeter of each colony (using a compass and a measuring tape to register the direction and distance between selected bordering trees where monarchs are clustering). These polygonal perimeter data are then processed with GIS (ArcView 3.3) to integrate and analyze the area for each colony. The enclosed area is calculated in hectares. The total area reported each year corresponds to the sum of the estimates obtained for each known monarch aggregation (locally named colony).

This work is conducted by the Comision Nacional de Areas Protegidas (CONANP) and WWF, with the support of the Alliance WWF Fundación Telmex Telcel. For details on this process, see WWF publication "Area of Forest Occupied by the Colonies of Monarch Butterflies in Mexico during the 2022-2023 Overwintering Period."

While population estimates are recorded back to the winter of 1976-1977, long term counts of monarchs previously occupying the overwintering sites for comparison are limited due to a lack of complete data.

Researchers have estimated that there are approximately 21.1 million butterflies per hectare, although this number most certainly varies with the time of the winter as the colonies contract, expand, and move. It also varies with the density and size of the trees in the colony. Based on this estimate the largest population of monarchs occurred in 1996-1997 when the colonies covered over 18 hectares and contained an estimated 380 million butterflies. To date the lowest population recorded was in 2013-2014 with 0.67 hectares and approximately 14 million monarchs.

Scientists have estimated that at least 6 hectares of overwintering area occupied is required to sustain the eastern monarch population.

Monarchs go through four phases while overwintering in Mexico: their arrival, the establishment of an overwintering colony, colony movement and finally the spring dispersal.

Arrival: Migrating monarchs usually arrive at the overwintering sites in late October through mid-November. In this early phase, the monarchs are largely scattered and diffuse in their flight, moving frequently through an area and eventually creating small clusters at night, while still continuing to move through the forest. During the day, their movement is common and widespread, as they search for the perfect sheltered location to spend the winter.

Establishment: As temperatures dip colder, monarchs begin to form larger and denser clusters, settling into smaller and protected areas at elevations of 2900-3300 meters (9,500 to 10,800 feet.) This usually occurs from mid-December through early February. The monarchs principally roost in oyamel trees although they use pines and other trees as well. This is the coldest time of the year, when monarchs are most compact and stationary in their clusters, a time of winter with little movement.

Movement: By mid-February, temperatures are gradually climbing and the monarchs begin expanding their clusters. They slowly begin to move down the mountain on warm, sunny days searching for water to drink in nearby creeks. They return to the safety of the nearby forest as temperatures drop. Mating also may be observed at this time.

Dispersal: The final phase is the monarch dispersal as the population leaves the colonies mid-March and gradually begins its movement north.

The traditional time of the annual overwintering count in Mexico is in late December when the clusters are most compact and movement is minimal. While counts continue biweekly during the time the monarchs are in the area, the end of December counts are used for comparison from year to year.

Note: This summary is modified from an article originally written by MBF Board members Gail Morris, Lincoln Brower and Karen Oberhauser that was published by the Monarch Joint Venture in 2017. It also includes input from MBF Board member Alfonso Alonso.

Critical Forest Habitat

The forests in Mexico provide unique microclimatic conditions that allow monarchs to survive the winter. Forest degradation is one factor putting monarchs and their migration in peril.

Sierra Chincua
Forest in Sierra Chincua, courtesy Fernanda Naomi Shimizu Romero

MBF supports work of local experts to help conserve these sensitive overwintering sites while also supporting local community and landowners. Please visit Our Work pages to learn more, including how you can help.

North American Monarch Conservation - Mexico, United States, Canada

Overwintering habitat in Mexico is only one link in a chain of critical habitats required by monarchs to survive and thrive. Milkweed and nectar are necessary throughout the insect's breeding range. Monarch habitat spans three countries, Mexico, the United States, and Canada.

In 2008, the Commission for Environmental Cooperation published a multi-national, multi-stakeholder, collaborative conservation plan (North American Monarch Conservation Plan - NAMCP) with the aim of maintaining healthy monarch populations and habitats throughout the migration flyways.

The U.S. based Monarch Joint Venture maintains the Monarch Conservation Implementation Plan as a more detailed guiding document for United States stakeholders to carry out the objectives in the NAMCP. The Monarch Joint Venture is a 501(c)3 non-profit dedicated protecting monarchs and their migration by collaborating with partners to deliver habitat conservation, education, and science across the United States.