Sexual segregation of gannet foraging over 11 years: movements vary but isotopic differences remain stable

Sex-specific niche differentiation is com- mon in marine vertebrates, but how this varies long- term is poorly understood. Here we investigated in- terannual variation in sexual segregation among breeding northern gannets Morus bassanus, wide- ranging central-place foragers with slight sexual di- morphism. Over 11 breeding seasons, we used GPS tracking and/or stable isotopes to test for sex differ- ences in foraging trip characteristics (range, duration and timing); spatial distribution; habitat selection; and carbon and nitrogen isotopes in blood. When combin- ing data from all years, females foraged further and for longer than males, yet despite this, the foraging ar- eas of the sexes almost completely overlapped. Males and females selected foraging habitats that differed in terms of oceanography but not fishing vessel density. We also detected temporal segregation: females were more likely to be at sea during the day than at night, while males were more likely to be at sea during the night. However, foraging behaviour quantified by all GPS analyses varied interannually, with sex differences detected in some years but not others. Finally, males had consistently higher red blood cell δ13C and δ15N than females across all years, which was not driven by size dimorphism, instead likely by prey choice or very fine-scale habitat selection. We conclude that envi- ronmental variation influenced short-term sex differ- ences in movement, but sex differences in stable iso- topes that integrate behaviour over longer periods reveal more consistent differences. Our results suggest that inferences drawn from single-year studies may not relate to general patterns, highlighting the impor- tance of long-term studies and combining methods.