Isotopes (δH) in wings and stored lipids of fall migratory monarch butterflies (Danaus plexippus) provide insights into population structure and nectaring origins.

We measured stable H isotope (δH) concentrations in wing and stored body lipids of fall migrating monarch butterflies (Danaus plexippus) from the eastern North American subpopulation along a transect from southern Ontario to overwintering sites in central Mexico from 2019 to 2021. We analysed wing δH to infer natal origins using both a spatially explicit approach and a categorical approach which assigned origins to north, central and southern portions of the breeding range west of the Appalachians. The fall migration differed in composition annually likely reflecting differential production of monarchs latitudinally due to varying climate conditions. Surprisingly, the composition of overwintering monarchs indicated that northern breeding regions contributed between 30 and 55% of overwintering monarchs across years, estimates far higher than previously reported. We suggest that northern productions zones are becoming more important compared to earlier decades. Bulk lipid δH values showed patterns of general increase from Ontario through Oklahoma before declining again through to the overwintering sites, supporting that monarchs acquire nectar for lipid production en route and finally at the overwinter sites at high-altitude regions of central Mexico. We recommend that stable isotope monitoring, especially of overwintering monarchs, be conducted annually to track production zones contributing to annual recruitment.