Every winter, scientists visit Mexico's 12 traditional sanctuaries and measure the area of forest covered by monarchs. In winter 2018/2019, the colonies covered an area of 6.05 hectares (as of December, 2018). They measure area because estimates of individual butterflies in a colony vary too widely to be reliable. Such estimates range from 10 to 50 million monarchs per hectare.

Monarch Population Teaching Resources


Background

Every year monarchs migrate to Mexico from across eastern North America. They cluster by the millions in 12 mountain sanctuaries. Because the butterflies are gathered in one small region of Mexico, scientists have a chance to estimate the size of the entire migratory population once each year.

Explore authentic data through graphs and maps.

1. Explore how scientists estimate the size of the monarch population in Mexico's overwintering region each year.