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

Bethany L. Clark* (Corresponding Author), Sam L. Cox, Kelly M. Atkins, Stuart Bearhop, Anthony W. J. Bicknell, Thomas Bodey, Ian R. Cleasby, W. James Grecian, Keith C. Hamer, Benjamin R. Loveday, Peter I. Miller, Greg Morgan, Lisa Morgan, Jason Newton, Samantha C. Patrick, Kylie L. Scales, Richard B. Sherley, Freydís Vigfúsdóttir, Ewan D. Wakefield, Stephen C. Votier

*Corresponding author for this work

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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.
Original languageEnglish
Pages (from-to)1-16
Number of pages16
JournalMarine Ecology Progress Series
Publication statusPublished - 4 Mar 2021

Bibliographical note

Acknowledgements. Research was funded by the Natural Environment Research Council (GW4+ Doctoral Training Partnership studentship to B.L.C. [NE/L002434/1]; Standard grant to K.C.H., S.B. and S.C.V. [NE/H007466/1]; and New Investigators grant to S.C.V. [NE/G001014/1]), and the Euro- pean Union (Interreg CHARM III). We thank the Royal Society for the Protection of Birds for permission to work on Grass- holm. We thank Toby Doyle, David Pascall and Lena Wilfert for assistance in molecular sexing, and David Pascall for analytical advice. We thank Richard Phillips and Brendan Godley for comments on the manuscript. Satellite products were ac- quired and processed by the Natural Environment Research Council Earth Observation Data Acquisition and Analysis Service (NEODAAS). We thank Matthew Witt for help in accessing vessel activity data. Many people helped with field- work, including Tim Guilford, Claudia Stauss, Sylvie Vandana- beele, Nicola Childs, Pearl Costello, Rocio Moreno, Matthew Gummery, Lisa Sztukowski, Jana Jeglinski, Matthew Carter, Matthew Nicholson, Dimas Gianuca, Rhiannon Meier, Laura Zango, Kirsten Archibald, Jacob Gonzalez-Solis, Jen Tyler, Tommy Clay, Calum Laver, Melanie Wells, Zoe Deakin, Zoe Courchene, Richard Phillips, John Arnould, Emma Dwan, Jack Wright, Georgia Bardua, Paulo Catry, Sarah Parmor and Megan Francis. We thank the editor Kyle Elliott and 3 anony- mous reviewers for helping us to improve the manuscript. The GPS data are stored on the BirdLife International Sea- bird Tracking Database (


  • Sex-specific
  • ecological niche
  • stable isotopes
  • biologging
  • seabirds
  • central-place foraging
  • northern gannet
  • Morus bassanus


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