The overwintering colonies of monarch butterflies in Mexico represent a critical welfare bottleneck where millions of individuals cluster in a small area.
Monarchs at overwintering sites face welfare risks from cold exposure when trees are removed, storm events, and winter drought. The concentration of millions of individuals at a small number of sites means that a single extreme weather event or habitat disruption can affect a significant proportion of the total population. Individual butterflies in marginal overwintering conditions experience thermal stress and disrupted metabolic states needed for spring migration.