Population Status

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.

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.