Northern rockhopper penguins prioritise future reproduction over chick provisioning

As iteroparous species, seabirds must balance present against future reproduction. We used stable-isotope (SIA) and stomach content analysis (SCA) to examine the effects of sex, breeding stage, pre-moulting period, ontogeny and chick age on the diet of northern rockhopper penguins (NRP; Eudyptes moseleyi) breeding at the Tristan da Cunha archipelago during 2010. Stomach contents were obtained from birds during the guard and creche stages. delta N-15 and delta C-13 signatures of whole blood of adults were measured at Tristan Island (during the incubation, guard and creche stages), and of adult feathers at Tristan and nearby (similar to 38 km) Nightingale Islands. delta N-15 and delta C-13 signatures of the whole blood of chicks were also measured during both stages. Adult pre-moult diets were significantly enriched in both elements at Nightingale compared to Tristan Islands. Adult female diet was dominated by zooplankton during the guard stage, and in both sexes by fish (predominantly Photichthyidae) in the creche stage. No effects of chick size within the guard stage, or of sex within the creche stage were observed on diet composition of adult birds. delta C-13 values of adult blood were higher in females relative to chicks, and in creche relative to guard stage individuals. delta N-15 values of blood showed a significant breeding stage x chick age interaction. Sex affected blood delta N-15 and delta C-13 only in the guard and incubation stages. Stage and sex interacted significantly on delta C-13 and delta N-15 signatures of adult feathers. NRP were opportunistic foragers, hunting in different areas during breeding and non-breeding periods, with different pre-moult foraging patterns observed in adults from colonies separated by only 38 km. Adults seemed to favour future reproduction, through brood reduction and by feeding chicks lower trophic level prey than they consumed themselves, although provisioning shifted from zooplankton to fish later in the breeding season.