Monarch butterfly overwintering sites in Mexico and California are critical to population survival. The welfare of millions of individuals clustering at these sites depends on forest protection and climate stability.
Overwintering monarch welfare is threatened by the degradation of the specific forest microhabitats their survival depends on. Monarchs in dense clusters require forest that maintains humidity and temperature within a narrow range — degraded forests with fewer trees expose clustered butterflies to temperature extremes that increase metabolic costs and mortality. Storm events in warming climates deliver more intense cold and rain to overwintering sites, causing mass mortality events where millions of individuals die from cold and wet conditions. The welfare significance of these events — at the scale of millions of individuals — is substantial even given uncertainty about insect sentience. Protecting overwintering forest integrity and addressing climate change are the most direct welfare interventions for overwintering monarchs.