Migration strategies of skuas in the southwest Atlantic Ocean revealed by stable isotopes

Abstract

Stable isotope ratios of carbon (δ13C) and nitrogen (δ15N) were measured in feathers to compare the non-breeding distributions and habitat use of adult brown skuas Stercorarius antarcticus lönnbergi from high-latitude colonies at Esperanza/Hope Bay (Antarctic Peninsula, 63°S) and Signy Island (South Orkneys, 60°S), with those from Bird Island (South Georgia, 54°S), which have also been tracked previously using geolocators. Breeding colony, but not sex, had a significant effect on feather δ13C and δ15N values. Feather stable isotope data from South Georgia birds mostly corresponded to oceanic, mixed subtropical–subantarctic to subantarctic waters, which agrees with the tracking data, as did a subset of the birds from the two higher latitude populations. However, other individuals displayed feather stable isotope ratios that were consistent with continental shelf or shelf-slope waters, suggesting that unlike the vast majority of brown skuas from South Georgia, many birds from higher latitude colonies spend the non-breeding season on, or near, the Patagonian Shelf. These population-level differences may have implications for exposure to anthropogenic threats or have carryover effects on subsequent breeding behaviour or performance.

Mercury Concentrations in Feathers of Albatrosses and Large Petrels at South Georgia: Contemporary Patterns and Comparisons with Past Decades

“…The high feather δ 13 C and δ 15 N values of white-chinned petrels and black-browed albatrosses are indicative of feeding in continental shelf and shelf-slope regions (Fig. 2 ) (Phillips et al 2009 ; Mills et al 2024 ), with the former feeding on the Patagonian Shelf and Humboldt Upwelling System off Chile during the non-breeding period (Phillips et al 2006 ), and the latter migrating to the Benguela Upwelling System off southwest Africa (Phillips et al 2005 ). Many of the birds that migrate to continental shelf and shelf-slope waters will follow vessels, feeding on discarded demersal fishes which potentially have a high Hg content, possibly because they have longer lifespans and feed at higher trophic levels on prey that usually have higher Hg levels than those in the epipelagic zone (Arcos et al 2002 ; Petersen et al 2008 ).…”

“…However, baseline δ 15 N values also vary spatially, which can obscure variation associated with trophic position (Elliott et al 2021 ). For instance, high δ 13 C and δ 15 N values of feathers from adult seabirds in the southwest Atlantic Ocean indicate that they feed in neritic waters (Phillips et al 2009 ; Mills et al 2024 ). Stable isotope values of feathers reflect diet during their synthesis, and because they are metabolically inert, they retain this information indefinitely (Cherel et al 2000 ).…”