Winter Monarch counts for 2021-22 were released this past May 2022.

Overwintering Monarchs Butterflies occupied an area of 2.85 hectares in Mexico in winter 2021-2022. This count happens every year when all the Monarchs have returned to the Oyamel Fir Forest area in the Butterfly Biosphere reserve.

This recent value of 2.85 hectares is a high given the low counts we have seen in recent years. For example, the 2020-2021 value was 2.1 hectares.

How Do You Count Monarchs?

western monarchs californiaInstead of counting individual Monarch Butterflies (very challenging), an estimate is made of the hectares of clumped Monarchs. While this may seem a strange unit to count, it seems to be the best way to count a 3D clump of millions of Monarchs.

How would you attempt to count these Monarchs? The count is often in late December when the clusters are compact and easier to count. They estimate there are 21.1 million butterflies per hectare! Yearly counts can then be compared on an ongoing basis (see figure below).

While this increase is something to celebrate, it is still far from the estimated value of 6ha needed to keep the population secure overall.

(Source: USGS Open File report 2020-1150, Summary of Available Data from the Monarch Overwintering Colonies in Central Mexico, 1976-1991)

This overwintering habitat must consist of grove conditions that include temperatures above freezing, but not too warm, low wind speeds, and access to fresh water (open water or fog drip or dew). A diversity of tree species is also thought to be important. These forests need to be conserved for the Monarch. Luckily, there are many conservation organizations undertaking local initiatives to encourage the forest habitat in this area of Mexico.

We are hopeful that the population can increase. We can help by planting native meadow habitats on rights-of ways, gardens, parks and open spaces.

Learn more about how CWF is working to Help the Monarchs