When it Comes to Winter Habitat, Monarchs are Choosy
Each year, thousands of monarch butterflies west of the Rockies migrate to the California coast, clustering on the open branches of trees to roost for the winter. Thanks to long-running efforts to count and track yearly monarch populations at hundreds of tree groves in coastal California, scientists have a good idea of how their numbers have changed over time and across locations. In recent years, they’ve seen periods of substantial declines, raising concerns about potential population collapse.
Researchers funded in part by the Southwest CASC are aiming to understand more about these declines and gain insight into what characteristics contribute to successful overwintering sites. In a recent study, the research team combined climate data with monarch counts at both the tree and grove scale across multiple overwintering locations in coastal California to explore how previous year population density, climate patterns, and tree composition might impact where monarchs seek winter habitat.
Their findings showed that monarchs can be selective, roosting only in specific coastal groves and on specific trees within those groves. Climate patterns appear to play a role at the regional scale, as monarchs avoided sites that experienced heavy storms (which can knock down branches and destroy tree habitat). Drought also plays a role, as spring drought and extremely low precipitation during fall migrations lowered overwintering abundance. Monarchs instead favored locations with moderate rainfall. Broader regional climate patterns were less significant within groves. Here, most monarchs consistently clustered on large, interior trees with variable shade that were well protected from harsh weather, highlighting how tree structure and location within the site might create ideal microclimates for wintering monarchs.
The study emphasizes the importance of protecting specific coastal trees that create reliable habitat, and maintaining structural diversity within groves so that they are more resilient to winter storms. Safeguarding sites that naturally buffer against extreme climate conditions can also support and maintain populations throughout the winter. The insights gained from this research can assist management efforts by helping to identify and prioritize habitat features that support overwintering monarchs during their yearly cluster along the California coast.
Read the full publication online at Conservation Science and Practice. Field photo credits: Peter Ibsen, USGS.