Reconstructing the historical wintering destinations of seabirds using stable isotope analyses of museum specimens

Stable isotope analyses of museum specimens provide an opportunity to examine long-term changes in avian migration strategies. Here, we determined stable isotopes of carbon (δ13C) and nitrogen (δ15N) in body feathers (reflecting diet during the non-breeding period) sampled from living birds and historical museum specimens to investigate whether brown skuas Stercorarius antarcticus lonnbergi from South Georgia and the South Shetland Islands changed their migration strategies during the past 100 years. Feathers sampled from adults in the 2010s at South Georgia had significantly lower δ13C and δ15N values than feathers from the 1910s, potentially indicating a shift from oceanic subtropical and continental shelf-slope waters towards mixed subtropical–subantarctic to subantarctic waters in the Argentine Basin. No isotope values (modern nor historical) from South Georgia were consistent with wintering on the Patagonian Shelf (rather than the shelf-slope). Feathers sampled at the South Shetland Islands in the 2020s had significantly lower δ13C and δ15N values than specimens collected 60–100 years previously. A plausible explanation here is reduced reliance on continental shelves and increased use of mixed subtropical–subantarctic to subantarctic waters as wintering destinations over time. Overall, our study highlights how museum collections can be used to investigate changing migration strategies of marine predators, including in remote oceanic ecosystems.