Monarch Population Trends

Eastern Population

The eastern monarch population is surveyed each winter starting in December when monarchs are clustered together. Counting individual monarchs in these sites is too challenging, so researchers estimate the population by measuring the area they occupy in hectares. Each hectare is approximately 2.47 acres, with the number of monarchs varying per hectare, but likely between 20-30 million monarchs per hectare (median 21.1, according to Thogmartin et al., 2017). Semmens et al., 2016 recommended a threshold of at least six hectares of overwintering monarchs to sustain a resilient eastern population and migration. The current population size remains lower than the target of 6 hectares, driving continued concern for the migration and necessitating increased conservation action.

Western Population

For over 25 years, The Xerces Society has led the Western Monarch Count, a community science effort dedicated to tracking the migratory western monarch butterfly population. Each year, volunteers and conservation organizations conduct surveys at overwintering sites along the Pacific coast—from Mendocino County, California, to Northern Baja, Mexico—and at inland sites in Arizona and California’s Saline Valley. During the 2024-2025 season, over 400 volunteers and partners participated in the effort, surveying over 250 overwintering sites and recording a peak population of just 9,119 monarch butterflies.