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Journal of Ornithology

, Volume 154, Issue 1, pp 239–249 | Cite as

Stable isotopes reveal variable foraging behaviour in a colony of the Imperial Shag Phalacrocorax atriceps: differences between ages, sexes and years

  • Andreas MichalikEmail author
  • Rona A. R. McGill
  • Hendrika J. van Noordwijk
  • Juan F. Masello
  • Robert W. Furness
  • Till Eggers
  • Petra Quillfeldt
Original Article

Abstract

Stable isotope analysis (SIA) is used in the study of trophic relationships in food webs, being also a powerful tool for the study of intraspecific diet segregation. Unlike short-term data from most conventional diet studies, SIA can also provide information about times when seabirds stay out at sea and are thus not easily accessible. Imperial Shags Phalacrocorax atriceps are resident seabirds showing sexual dimorphism in body size. Previous studies showed strong intra-species dietary segregation between male and female Imperial Shags during the breeding season. Between 2006 and 2009 at New Island in the Falkland Islands/Islas Malvinas (southwest Atlantic), we investigated if intraspecific differences were also present between age-groups, namely adults and immature males. We further tested if differences existed over the year, particularly during the non-breeding season. Finally, we tested if differences were consistent among studied years. We found no differences in δ13C values between immature and adult males. However, lower δ15N values indicated feeding at lower trophic levels for immatures. This might be explained by poorer hunting abilities of young, inexperienced birds, compared to adults. In both the breeding and non-breeding seasons, differences among years in both δ13C and δ15N suggest that the foraging behaviour of the shags differed among years in terms of spatial distribution and trophic level. Males consistently foraged on a higher trophic level than females and had lower δ13C values, which is in line with the use of foraging areas further offshore.

Keywords

Diet Imperial Shag Phalacrocorax atriceps Patagonian shelf Stable isotopes 

Zusammenfassung

Stabile Isotope zeigen variables Ernährungsverhalten in einer Kolonie von Blauaugenscharben Phalacrocorax atriceps : Unterschiede zwischen Altersgruppen, Geschlechtern und Jahren

Analyse von stabilen Isotopen wird häufig in der Untersuchung von Nahrungsnetzen angewendet, kann aber auch eine Methode sein, intraspezifische Ernährungsunterschiede aufzudecken. Im Gegensatz zu den meisten Studien mit konventionellen Methoden können durch die Analyse stabiler Isotopen zudem Aussagen über Zeiträume getroffen werden, die Seevögel auf See verbringen und daher für Forscher schwer zugänglich sind. Blauaugenscharben Phalacrocorax atricpes sind residente Seevögel mit ausgeprägtem Sexualdimorphismus. Bisherige Studien zeigten starke intraspezifische Unterschiede zwischen männlichen und weiblichen Brutvögeln. In einer Kolonie auf den Falklandinseln wurde zwischen 2006 und 2009 untersucht, ob sich auch intraspezifische Ernährungsunterschiede zwischen immaturen und adulten Männchen nachweisen lassen. Außerdem wurde getestet, ob die Unterschiede auch außerhalb der Brutzeit Bestand haben. Schließlich wurden die drei Untersuchungsjahre verglichen. Zwischen den Altersgruppen konnten keine Unterschiede in den δ13C-Werten gefunden werden, jedoch hatten Proben immaturer Männchen geringere δ15N-Werte. Dies deutet bei den immaturen Männchen auf kleinere Beutetiere hin, was durch deren schlechtere Jagdfähigkeiten erklärt werden kann. Sowohl innerhalb als auch außerhalb der Brutzeit sowie zwischen den Jahren wurden Unterschiede zwischen den δ13C- und den δ15N-Werten gefunden. Dies zeigt, dass intraspezifische Ernährungsunterschiede auch außerhalb der Brutzeit bestehen bleiben und sich die Lage der Jagdgebiete sowie die Zusammensetzung der Nahrung von Jahr zu Jahr ändern können. Männchen erbeuten überwiegend größere Beutetiere als Weibchen. Sie weisen niedrigere δ13C-Werte als Weibchen auf und jagen küstenferner nach Nahrung.

Notes

Acknowledgments

We wish to thank to the New Island Conservation Trust, Ian, Maria and Georgina Strange and Dan Birch who facilitated fieldwork. Financial support for fieldwork came from DFG-German Research Foundation, (Qu 148/1-ff) and DAAD—the German Academic Exchange Service. Fieldwork at New Island was approved by the Falkland Islands Government (Environmental Planning Office). Funding for the stable isotope work was provided by the Natural Environment Research Council, UK.

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Copyright information

© Dt. Ornithologen-Gesellschaft e.V. 2012

Authors and Affiliations

  • Andreas Michalik
    • 1
    • 2
    Email author
  • Rona A. R. McGill
    • 3
  • Hendrika J. van Noordwijk
    • 1
  • Juan F. Masello
    • 1
    • 4
  • Robert W. Furness
    • 5
  • Till Eggers
    • 2
  • Petra Quillfeldt
    • 1
    • 4
  1. 1.Max-Planck-Institut für OrnithologieVogelwarte RadolfzellRadolfzellGermany
  2. 2.Department of Biology/Chemistry, Experimental Ecology GroupUniversity of OsnabrückOsnabrückGermany
  3. 3.Scottish Universities Environmental Research CentreGlasgowUK
  4. 4.Department of Animal Ecology and BiodiversityJustus Liebig University GiessenGiessenGermany
  5. 5.College of Medical, Veterinary and Life SciencesUniversity of GlasgowGlasgowUK

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