Diet specialization in a colonial seabird studied using three complementary dietary techniques: effects of intrinsic and extrinsic factors
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The breeding period has a critical influence on the trophic ecology of seabirds because of the energetic costs of egg production for females, the need to return regularly to the nest to provision chicks, the combined energetic demands of adults and chicks, and potential intraspecific competition if resources around the colony are scarce. The present study combined three dietary methods to investigate if and how these intrinsic and extrinsic factors influenced diet specialization in a colonially breeding seabird, the Cape gannet Morus capensis. The diet of this species was studied from November 2009 to October 2010 at the species’ largest colony at Bird Island, Algoa Bay (33°50′S, 026°17′E; South Africa). Potential prey species were sampled concurrently and dietary tracers (stable isotopes and fatty acids) were analysed. Stomach content and carbon and nitrogen stable isotope analyses indicated that adults relied heavily all year round on small pelagic fish (anchovy and sardine), with prey species composition and individual prey size changing with season, probably reflecting prey biology. Dietary tracers did not show any differences between adult and chick diets. Subtle differences were found between stable isotope values of adult males and females but these were not supported by a Bayesian mixing model. In contrast, differences between the sexes were highlighted in blood fatty acids. The combined results suggest that these were probably related to the cost of egg production rather than to inter-sex differences in diet. Individual diet specialization was observed using stable isotopes in adults. Altogether this dataset indicates the importance of combining complementary methods to understand multiple facets of seabirds’ trophic ecology, and highlights interactions with fisheries that require future monitoring.
The authors would like to thank George Kant from the Department of Agriculture, Forestry and Fisheries who assisted with the collection of fish samples, Jenny Booth and David Grémillet for their help during fieldwork, Rob Crawford for the updated Cape gannet population counts, and the South African National Parks for facilitating the research on Bird Island. Fatty acid analyses were conducted in the Fatty Acid Facility at Rhodes University funded through the National Research Foundation. Stable isotope analyses were conducted by Ian Newton under the supervision of John Lanham at the Stable Light Isotope Laboratory, University of Cape Town. This work is based on the research supported by the South African Research Chairs Initiative of the Department of Science and Technology and the National Research Foundation. This work was undertaken under an ethics permit granted by Rhodes University. The authors would like to thank the two anonymous reviewers for their constructive comments which greatly improved the manuscript.
Compliance with ethical standards
Conflict of interest
The authors declare that there is no conflict of interest and that consent was obtained from all parties.
All work on Cape gannets was performed under permits issued by the Ethic committee from Rhodes University and South African National Parks (CONM746).
- Bearhop S, Teece MA, Waldron S, Furness RW (2000) Influence of lipid and uric acid on δ13C and δ15N values of avian blood: implications for trophic studies. Auk 117:504–507. doi:10.1642/0004-8038(2000)117[0504:IOLAUA]2.0.CO;2Google Scholar
- Becker BH, Newman SH, Inglis S, Beissinger SR (2007) Diet-feather stable isotope (δ15N and δ13C) fractionation in common murres and other seabirds. Condor 109:451–456. doi:10.1650/0010-5422(2007)109[451:DSINAC]2.0.CO;2Google Scholar
- BirdLife International (2017a) Species factsheet: Morus capensis. http://www.birdlife.org. Accessed 24 Jan 2017
- BirdLife International (2017b) http://www.birdlife.org/globally-threatened-bird-forums/2017/06/cape-gannet-morus-capensis-uplist-to-endangered/ Accessed 14 June 2017
- Christie WW (1973) Lipid analysis: isolation, separation, identification and structural analysis of lipids. Pergamon Press, New YorkGoogle Scholar
- Clarke MR (1986) A handbook for the identification of cephalopod beaks. Clarendon Press, OxfordGoogle Scholar
- Crawford RJM, Whittington PA, Martin AP, Tree AJ, Makhado AB (2009) Population trends of seabirds breeding in South Africa’s Eastern Cape and the possible influence of anthropogenic and environmental change. Mar Ornithol 37:159–174Google Scholar
- Department of Agriculture, Forestry and Fisheries (2014) Status of the South African marine fishery resources 2014. Cape Town, South AfricaGoogle Scholar
- Folch J, Lees M, Stanley GHS (1957) A simple method for the isolation ad purification of total lipides from animal tissues. J Biol Chem 226:497–509Google Scholar
- Galloway AWE, Brett MT, Holtgrieve GW, Ward EJ, Ballantyne AP, Burns CW, Kainz MJ, Müller-Navarra DC, Persson J, Ravet JL, Strandberg U, Taipale SJ, Alhgren G (2015) A fatty acid based bayesian approach for inferring diet in aquatic consumers. PLoS One 10:e0129723. doi: 10.1371/journal.pone.0129723 CrossRefGoogle Scholar
- Hammer Ø, Harper DAT, Ryan PD (2001) PAST: palaeontological statistics software package for education and data analysis. Palaeontol Electron 4:9pGoogle Scholar
- Jaquemet S, Potier M, Cherel Y, Kojadinovic J, Bustamante P, Richard P, Catry P, Ramos JA, Le Corre M (2008) Comparative foraging ecology and ecological niche of a superabundant tropical seabird: the sooty tern Sterna fuscata in the southwest Indian Ocean. Mar Biol 155:505–520. doi: 10.1007/s00227-008-1049-1 CrossRefGoogle Scholar
- Moseley C, Grémillet D, Connan M, Ryan PG, Mullers RHE, van der Lingen CD, Miller TW, Coetzee JC, Crawford RJM, Sabarros P, McQuaid CD, Pichegru L (2012) Foraging ecology and ecophysiology of Cape gannets from colonies in contrasting feeding environments. J Exp Mar Biol Ecol 422–423:29–38. doi: 10.1016/j.jembe.2012.04.002 CrossRefGoogle Scholar
- Nelson JB (1965) The behaviour of the Gannet. Br Birds 58:233–288Google Scholar
- Nelson B (1980) Seabirds. Their biology and ecology. The Hamlyn Publishing Group Limited, FelthamGoogle Scholar
- Pichegru L, Ryan PG, van der Lingen CD, Coetzee J, Ropert-Coudert Y, Grémillet D (2007) Foraging behaviour and energetics of Cape gannets Morus capensis feeding on live prey and fishery discards in the Benguela upwelling system. Mar Ecol Prog Ser 350:127–136. doi: 10.3354/meps07128 CrossRefGoogle Scholar
- R Development Team (2015) A Language and Environment for Statistical Computing. R Foundation for Statistical Computing, Vienna, AustriaGoogle Scholar
- Smale MJ, Watson G, Hecht T (1995) African marine fishes. In: Ichthyological monographs of the JLB Smith Institute of Ichthyology, vol 1. JLB Smith Institute of Ichthyology, Grahamstown, pp 1–253Google Scholar
- Stauss C, Bearhop S, Bodey TW, Garthe S, Gunn C, Grecian WJ, Inger R, Knight ME, Newton J, Patrick SC, Phillips RA, Waggitt JJ, Votier SC (2012) Sex-specific foraging behaviour in northern gannets Morus bassanus: incidence and implications. Mar Ecol Prog Ser 457:151–162. doi: 10.3354/meps09734 CrossRefGoogle Scholar
- Stock BC and Semmens BX (2015) MixSIAR User Manual, version 3.0Google Scholar
- Tew Kai E, Benhamou S, van der Lingen CD, Coetzee JC, Pichegru L, Ryan PG, Gremillet D (2013) Are Cape gannets dependent on fishery waste? A multi-scale analysis using seabird GPS-tracking, hydroacoustic surveys of pelagic fish, and vessel monitoring systems. J Appl Ecol 50:659–670. doi: 10.1111/1365-2664.12086 CrossRefGoogle Scholar
- van der Lingen CD, Coetzee JC, Hutchings LF. 2011. Causes and effects of changes in the distribution of sardine and anchovy in shelf waters off South Africa. In: Zietsmnan L (ed) Observations on environmental change in South Africa. SUN MeDIA, Stellenbosch, pp 252–277. ISBN 978-1-920338024-4Google Scholar
- Weimerskirch H, Cherel Y, Cuenot-Chaillet F, Ridoux V (1997) Alternative foraging strategies and resource allocation by male and female wandering albatrosses. Ecology 78:2051–2063. doi:10.1890/0012-9658(1997)078[2051:AFSARA]2.0.CO;2Google Scholar
- Weimerskirch H, Barbraud C, Lys P (2000) Sex differences in parental investment and chick growth in Wandering albatrosses: fitness consequences. Ecology 81:309–318. doi:10.1890/0012-9658(2000)081[0309:SDIPIA]2.0.CO;2Google Scholar
- Williams TD (2005) Mechanisms underlying the costs of egg production. Bioscience 55:39–48. doi:10.1641/0006-3568(2005)055[0039:MUTCOE]2.0.CO;2Google Scholar