ALBERT

Description: Marine top predators forage in environments that show potentially extreme temporal and spatial variation in prey availability, with reproductive success being crucially linked to food supply. Multiple factors of interannual and sexual variation, as well as variation across breeding stages, can shape patterns of spatial use in foraging seabirds, yet studies that address all of these variables simultaneously are rare. We present spatial assessment of foraging patterns by µGPS tracking of a sexually size monomorphic, long-lived species, the Australasian gannet (Morus serrator). The study spanned the incubation and chick-rearing stages in three consecutive breeding seasons. Our findings revealed high interannual variability in foraging distances and trip durations, but no consistent differences between birds across different breeding stages or the sexes. The exception was that core foraging areas were different for female and male Australasian gannets, although trip durations or distances were similar for both sexes. Our results also indicate bimodality in foraging distance and trip duration in this species, while highlighting interannual variability in the extent of bimodality. These findings contribute to a scarcely documented type of foraging behaviour in the seabird family of the Sulidae. Overall, these spatial use patterns provide a baseline for understanding the evolution of sex-specific foraging differences in biparental seabirds, and the extent to which these differences might help in securing breeding success across years of variable food availability.

Description: Sex differences in foraging behaviour are typically studied in size-dimorphic taxa. Data on sex-specific behavior in monomorphic taxa are needed to test theories of reproductive investment. It has been suggested that in seabirds foraging niche separation may be related to decreased intersexual competition for food between cooperating pair-bonded individuals. Alternatively, sex differences in foraging niches may be driven by different nutritional requirements of females associated with the reproductive costs of egg production and oviposition. To assess these possibilities, we studied a size-monomorphic colonial seabird, the Australasian Gannet (Morus serrator) at the Cape Kidnappers gannetry, New Zealand. We recorded maximum dive depths, and distinct diet composition of incubating females as indicated by stable isotopic signatures. Results suggested greater female foraging effort during early times of incubation, indicated by significantly deeper maximum dives. Sex-specific foraging patterns across other breeding stages were more variable. Nitrogen stable isotopic values showed that incubating females occupied a different trophic position compared to males at the same breeding stage, and also from those of gannets of both sexes at later stages of parental care. Overall, the data are consistent with cost-of-oviposition compensation in females necessitating male-bias in parental care in biparental breeders. Further research is needed to unravel the implications for the evolution of sex differences in behavior in this and other monomorphic taxa.

Description: Individual foraging niche specialisation is recognised as an important determinant of ecological structure and function, particularly for species with high per-capita trophic influence like marine apex predators. Stable isotopes provide a powerful approach to understand the mechanisms and consequences of niche specialisation, especially in cryptic predators, yet have poor resolution for incorporating nutrition, which is a critical aspect of foraging interactions. Multidimensional nutritional ecology compartmentalises foods and diets as mixtures of nutrients (e.g. proteins, lipids, carbohydrates) to draw mechanistic links between individual nutrition, physiology, behaviour and fitness, the drivers of foraging decisions and their trophic implications. Yet difficulties in establishing time-integrated nutrient intakes has hampered progress in predator nutritional ecology. We employed a novel approach integrating stable isotopes and a multidimensional nutritional niche framework to investigate individual diet specialisation across multiple niche realisations in juvenile white sharks (Carcharodon carcharias). We sampled tooth files to generate individual isotopic profiles (δ13C, δ15N, δ34S) and characterise individual isotopic niches (δ-space). We then contextualised isotopic specialisation patterns by estimating individual prey use (p-space) using Bayesian mixing models, and further combined this with nutritional geometry framework models to quantify the nutritional dimension (N-space) of individual specialisation. Isotopic analyses indicated that juvenile white sharks mostly foraged as individual specialists within a broader, generalist, population niche. Mixing models showed individual differences in the consumption of dolphin, shark and batoid prey (rays), which comprised much of the overall diet. Despite δ-space and p-space variance, modelled nutrient intakes were similar for most sharks, which may suggest complementary feeding on nutritionally disparate prey among individual sharks towards a particular nutritional goal. Our approach helps to address the challenges of estimating time-integrated nutrient intakes in cryptic species, expanding our knowledge of individual diet specialisation into a nutritional dimension and revealing how specialisations may differ across niche realisations (prey use and nutritional niches). We argue that this can facilitate stronger links between individual diet specialisation, its intrinsic (e.g. nutritional goals, fitness) and extrinsic (e.g. trophic dynamics) outcomes.