Apparent mismatch between stable isotopes and foraging habitat suggests high secondary ingestion of Antarctic krill in brown skuas

Abstract

Changes in seasonal resource availability and in energetic requirements as offspring grow may force parents to change their trophic ecology throughout the breeding season. Brown skuas Stercorarius antarcticus breed in a highly seasonal environment where the availability of their main food resource changes during the season. We studied the feeding plasticity of breeding brown skuas by assessing their isotopic diet and movement patterns at different stages of their breeding cycle. Blood δ15N values indicated that penguin chicks Pygoscelis spp. and Antarctic krill Euphausia superba constituted most of the diet of brown skuas (up to ~70%), and that there was an increase in the ingestion of lower trophic level prey (most likely Antarctic krill) throughout the breeding period (from 30-46%). This contrasts with movement analysis indicating that 65% of the overall foraging locations were within penguin rookeries. The apparent contradiction between the results from both techniques may be explained by a combination of some skuas feeding mostly at sea or on shore together with secondary ingestion of marine resources from the penguins’ gut by feeding mostly within penguin rookeries. Krill obtained in that way may provide protein to replenish reserves before migration along with globulins through the intake of carotenoids. These results highlight the fact that the trophic ecology of species can be more complex than that suggested by one single method and emphasizes the importance of combining techniques to draw robust conclusions. In addition, our study indicates that skuas may select portions of prey to obtain specific resources to fulfill their nutritional requirements.