Integrating Geolocator Tracking and Isotopic Tools to Reveal Winter Foraging Ecology and Mercury Exposure in Arctic Seabirds

Winter is a critical yet understudied phase in the annual cycle of Arctic seabirds, largely due to logistical challenges of polar fieldwork. While geolocators have advanced our understanding of migration and overwintering behavior, their cost and technical limitations constrain widespread use. As a complementary and scalable alternative, feather analysis offers integrated insights into both ecology and contaminant exposure at individual and population levels.

In this study, we examined head feathers from thick-billed murres (Uria lomvia) collected at seven colonies spanning West Greenland, the Canadian Arctic, and Svalbard. This species breeds widely across the circumpolar Arctic, but several Atlantic populations are in decline. Because head feathers are grown during the non-breeding season, they reflect mercury exposure at overwintering sites. We measured total mercury concentrations, stable isotopes of carbon (δ¹³C) and nitrogen (δ¹⁵N), and mercury isotope compositions (δ²⁰²Hg, Δ¹⁹⁹Hg) to assess variation in winter foraging habitats and mercury exposure pathways. Our results reveal distinct spatial patterns in δ²⁰²Hg that align with known west-to-east gradients in the Hg isotopic composition of North Atlantic prey fish, suggesting region-specific foraging areas during winter. Intra-colony variation in δ²⁰²Hg further highlights individual-level differences in winter habitat use, consistent with patterns derived from geolocator data. Additionally, the strong positive correlation between total Hg concentration and Δ¹⁹⁹Hg suggests that foraging depth significantly influences mercury uptake. These findings demonstrate that an integrated isotopic-tracking approach advances ecological biogeochemistry by tracing both contaminant pathways and seabird movement using natural isotopic tracers.