A collaborative research paper (led by Sarah Saunders and Elise Zipkin of Michigan State University) showing that monarch overwinter colony size is still primarily driven by summer population size, but that only nectar availability during fall migration and loss of forest habitat explains their continued declines over the last 15 years (note that this does not include their greatest period of decline during the 1990s). Surprisingly, disease did not contribute to declining numbers during the fall. This study helps to resolve some of the controversy regarding the disconnect between summer and winter population trajectories. Summer populations have not shown a decline over time, while overwinter sites have. This study shows that, at least over the last 15 years, a combination of factors during fall migration is contributing to the decline. This work represents a larger collaboration with Mexican colleagues who work at the Monarch Butterfly Biosphere Reserve, including Eduardo Rendon from WWF, Isabella Ramirez from UNAM, and Eligio Garcia-Serrano from the Monarch fund.