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Marine Ecology Progress Series

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MEPS 661:1-16 (2021)  -  DOI:

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

Bethany L. Clark1,2,*, Sam L. Cox3,4,5,6, Kelly M. Atkins1, Stuart Bearhop7, Anthony W. J. Bicknell8, Thomas W. Bodey1,9, Ian R. Cleasby10, W. James Grecian11, Keith C. Hamer12, Benjamin R. Loveday13, Peter I. Miller13, Greg Morgan14, Lisa Morgan15, Jason Newton16, Samantha C. Patrick17, Kylie L. Scales18, Richard B. Sherley7, Freydís Vigfúsdóttir19, Ewan D. Wakefield20, Stephen C. Votier1,21

1Environment and Sustainability Institute, University of Exeter, Penryn TR10 9FE, UK
2BirdLife International, The David Attenborough Building, Pembroke Street, Cambridge CB2 3QZ, UK
Addresses for other authors are given in the Supplement at
*Corresponding author:

ABSTRACT: Sex-specific niche differentiation is common in marine vertebrates, but how this varies long-term is poorly understood. Here we investigated interannual variation in sexual segregation among breeding northern gannets Morus bassanus, wide-ranging central-place foragers with slight sexual dimorphism. Over 11 breeding seasons, we used GPS tracking and/or stable isotopes to test for sex differences in foraging trip characteristics (range, duration and timing); spatial distribution; habitat selection; and carbon and nitrogen isotopes in blood. When combining data from all years, females foraged further and for longer than males, yet despite this, the foraging areas 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 environmental variation influenced short-term sex differences in movement, but sex differences in stable isotopes 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 importance of long-term studies and combining methods.

KEY WORDS: Sex-specific · Ecological niche · Stable isotopes · Biologging · Seabird · Central-place foraging · Northern gannet · Morus bassanus

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Cite this article as: Clark BL, Cox SL, Atkins KM, Bearhop S and others (2021) Sexual segregation of gannet foraging over 11 years: movements vary but isotopic differences remain stable. Mar Ecol Prog Ser 661:1-16.

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