The overwintering colonies of eastern monarch butterflies in Mexican oyamel fir forests represent one of nature's most extraordinary aggregations, but these colonies face logging, climate, and tourism pressures that threaten millions of individuals.
Monarch overwintering colonies face welfare threats from temperature extremes. Monarchs are cold-blooded and maintain metabolic torpor during winter, clustered on trees for mutual warmth. When extreme weather events - cold rain or snow - penetrate the forest buffer, millions of butterflies fall from trees and die from hypothermia. Whether this mass death constitutes welfare harm in a morally significant sense depends on uncertain questions about butterfly sentience, but the scale and preventable nature of these events make them worth consideration. Logging that reduces forest buffer exposes colonies to temperature extremes that cause mass mortality. Tourism pressure that flushes colonies from trees during cold periods forces metabolic expenditure that depletes fat reserves needed for spring migration.