Abstract
Northern Macronectes halli and southern giant petrels M. giganteus are opportunistic predators and the dominant avian scavengers in sub-Antarctic and Antarctic ecosystems. At South Georgia, there are globally important breeding populations of both species; however, no detailed diet study has been carried out at this site for > 35 years. Here, we analysed stomach contents of northern (n = 81) and southern giant petrel (n = 73) chicks at Bird Island, South Georgia (2014/15–2016/17). Specifically, we investigated: (1) interspecific and sexual dietary segregation; (2) diet changes within and among recent years; and (3) long-term diet changes since 1979/80–1980/81. Overall diet composition was similar between species, with penguins comprising approximately half the diet by mass. In both species (but more pronounced in southern giant petrels), prey delivered by female parents included higher proportions (by mass) of penguins and Antarctic krill Euphausia superba, and by male parents included more Antarctic fur seal Arctocephalus gazella carrion and seabirds other than penguins. Consumption of penguins increased, and of seal carrion declined, as chick-rearing progressed, mirroring the decreasing availability of the latter after the peak pupping period in December at South Georgia. Comparisons with data from 1979/80–1980/81 suggest some changes in giant petrel diets; however, these differences were no greater than the typical annual variation, and there were no clear links between diet and breeding allochrony or the differing population trends (more rapid increase in northern than southern giant petrels). The high diet diversity and ability of giant petrels to exploit both carrion and marine prey facilitates coexistence with other sympatric Procellariiformes and explains their favourable population status.
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Acknowledgements
The authors are grateful to Alastair Wilson, Jerry Gilham, Tegan Newman and the other fieldworkers at Bird Island who helped with sampling of giant petrel diets, and to Dr José Xavier for identification of cephalopod beaks. Constructive comments from four anonymous reviewers substantially improved this work. W.F.M was supported by a NERC GW4+ Doctoral Training Partnership studentship from the Natural Environment Research Council (NE/L002434/1). This work represents a contribution to the Ecosystems component of the British Antarctic Survey Polar Science for Planet Earth Programme, funded by the Natural Environment Research Council.
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WFM and RAP contributed to the concept and design of the study. TIM collected samples in the field. WFM and TIM analysed the data. WFM wrote the initial manuscript draft. All authors contributed to the writing of the manuscript and gave final approval for publication.
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Diet sampling was approved by the British Antarctic Survey Ethics Committee and carried out with permission of the Government of South Georgia and the South Sandwich Islands.
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Mills, W.F., Morley, T.I., Votier, S.C. et al. Long-term inter- and intraspecific dietary variation in sibling seabird species. Mar Biol 168, 31 (2021). https://doi.org/10.1007/s00227-021-03839-6
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DOI: https://doi.org/10.1007/s00227-021-03839-6