Using museum specimens to track morphological shifts through climate change

over the past 60 years within three North American geographical regions. We find declines in ventral wing melanism, which correspond to reduced absorption of solar radiation and thus reduced risk of overheating, in two regions. However, contrary to expected responses to climate warming, we find melanism increases in the most thoroughly sampled region. Relationships among temperature, phenology and morphology vary across years and complicate the distinction between plastic and genetic responses. Differences in these relationships may account for the differing morphological shifts among regions. Our findings highlight the promise of using museum specimens to test mechanistic hypotheses for shifts in functional traits, which is essential for deciphering interacting responses to climate change.This article is part of the theme issue 'Biological collections for understanding biodiversity in the Anthropocene'.