When viewed over a period of years, the frequency and type of seasonal occupancy of overwintering monarch butterflies, Danaus plexippus plexippus (Linnaeus, 1758), at California winter groves are good indicators of whether grove conditions are being maintained, declining, or recovering. The contribution of a winter grove to the next generation of butterflies depends on the stability of grove conditions for winter aggregations. Knowing the trend of grove conditions allows proactive management of winter groves for overwintering butterflies. [ABSTRACT FROM AUTHOR]