Inter- and intra-individual variation in the diet of Australasian gannets Morus serrator

Animal diets often vary according to age, sex, experience and/or individual preferences, which, when maintained over time, can lead to behavioural consistency and individual specialisations within populations. In addition, behavioural and dietary similarity within breeding pairs confers reproductive benefits in some species. We investigated inter- and intra-individual variation in diet in Australasian gannets Morus serrator through analysis of voluntary regurgitations, blood plasma stable isotopes and reconstructed diets. Samples were collected from nesting adults (mostly partners) over 4 breeding seasons (2012-2015) at 2 colonies (Point Danger, PD; and Pope’s Eye, PE), 215 km apart and with divergent oceanographic conditions. Inter-individual variation in δ13C and δ15N values and reconstructed diets was associated with colony, year, breeding stage and sex. The diet of PD individuals was dominated by pelagic schooling prey species, whereas PE birds consumed a substantial amount of benthic/inshore species. Correspondingly, the proportional similarity in diet of individuals was greater at PD, where individuals foraged within a relatively uniform environment, than at PE, where birds had access to a greater diversity of foraging habitats. Intra-individual variation in isotopic values indicated that trophic consistency was higher over medium timescales (between breeding stages within breeding seasons) than longer timescales (between breeding seasons), in accordance with recently documented temporal patterns of behavioural consistency. Lastly, nest partners consumed prey of similar trophic level (δ15N values), although a high degree of similarity did not confer a reproductive advantage to nest partners, and the mechanisms for such similarity are unknown.