MEPS 486:289-304 (2013)  -  DOI: https://doi.org/10.3354/meps10371

ABSTRACT: 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 crèche stages. δ¹⁵N and δ¹³C signatures of whole blood of adults were measured at Tristan Island (during the incubation, guard and crèche stages), and of adult feathers at Tristan and nearby (~38 km) Nightingale Islands. δ¹⁵N and δ¹³C 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 crèche stage. No effects of chick size within the guard stage, or of sex within the crèche stage were observed on diet composition of adult birds. δ¹³C values of adult blood were higher in females relative to chicks, and in crèche relative to guard stage individuals. δ¹⁵N values of blood showed a significant breeding stage × chick age interaction. Sex affected blood δ¹⁵N and δ¹³C only in the guard and incubation stages. Stage and sex interacted significantly on δ¹³C and δ¹⁵N 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.

KEY WORDS: Sex-specific foraging · Selective chick provisioning · Breeding stage · Stable isotope analysis · Stomach content analysis · Northern rockhopper penguins