Behavioural strategies of cormorants (Phalacrocoracidae) foraging under challenging light conditions

Authors

  • CRAIG R. WHITE,

    Corresponding author
    1. Centre for Ornithology, School of Biosciences, The University of Birmingham, Edgbaston, Birmingham B15 2TT, UK
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    • Present address: School of Integrative Biology, The University of Queensland, St Lucia, Queensland, Australia, 4072.

  • PATRICK J. BUTLER,

    1. Centre for Ornithology, School of Biosciences, The University of Birmingham, Edgbaston, Birmingham B15 2TT, UK
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  • DAVID GRÉMILLET,

    1. Centre National de la Recherche Scientifique, DEPE-IPHC, 23 rue Becquerel, F-67087 Strasbourg Cedex 02, France
    2. DST/NRF Centre of Excellence at the Percy FitzPatrick Institute, University of Cape Town, Rondebosch 7701, South Africa
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    • #

      Present address: CEFE-CNRS, 1919 Route de Mende, F-34293 Montpellier, France.

  • GRAHAM R. MARTIN

    1. Centre for Ornithology, School of Biosciences, The University of Birmingham, Edgbaston, Birmingham B15 2TT, UK
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  • Conflict of interests: The authors declare no conflict of interests.

*Corresponding author.
Email: craig.white@uq.edu.au

Abstract

Diving is indicative of foraging in cormorants (Phalacrocoracidae). We have investigated a range of parameters associated with diving in Great Cormorants Phalacrocorax carbo to provide insight into the bases of cormorant predatory strategies. We hypothesize that if vision is important in cormorant foraging behaviour, and if they are not constrained by the position of their prey in the water column, then diving behaviour will be modulated primarily in response to the diel variation in ambient light levels. Specifically, we propose that cormorants forage at shallower depths when light levels are low, and more deeply when light levels are high. We provide evidence that this is the case. We recorded the occurrence of cormorant diving behaviour using implanted data loggers and recorded ambient light levels and water temperature using leg-mounted loggers in a sample of free-living Great Cormorants in Greenland. Our results show that dives are shallower at the beginning and end of each day when light levels are lower. We suggest that these data support the hypothesis that cormorant foraging is visually-guided even though recent evidence has shown that their underwater visual acuity is poor.

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