Digestive bottleneck affects foraging decisions in red knots Calidris canutus. I. Prey choice

Authors

  • JAN A. VAN GILS,

    Corresponding author
    1. Department of Marine Ecology and Evolution, Royal Netherlands Institute for Sea Research (NIOZ), PO Box 59, 1790 AB Den Burg, Texel, the Netherlands; and
    2. Animal Ecology Group, Centre for Ecological and Evolutionary Studies (CEES), University of Groningen, PO Box 14, 9750 AA Haren, the Netherlands
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  • SEM R. DE ROOIJ,

    1. Department of Marine Ecology and Evolution, Royal Netherlands Institute for Sea Research (NIOZ), PO Box 59, 1790 AB Den Burg, Texel, the Netherlands; and
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  • JELMER VAN BELLE,

    1. Department of Marine Ecology and Evolution, Royal Netherlands Institute for Sea Research (NIOZ), PO Box 59, 1790 AB Den Burg, Texel, the Netherlands; and
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  • JAAP VAN DER MEER,

    1. Department of Marine Ecology and Evolution, Royal Netherlands Institute for Sea Research (NIOZ), PO Box 59, 1790 AB Den Burg, Texel, the Netherlands; and
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  • ANNE DEKINGA,

    1. Department of Marine Ecology and Evolution, Royal Netherlands Institute for Sea Research (NIOZ), PO Box 59, 1790 AB Den Burg, Texel, the Netherlands; and
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  • THEUNIS PIERSMA,

    1. Department of Marine Ecology and Evolution, Royal Netherlands Institute for Sea Research (NIOZ), PO Box 59, 1790 AB Den Burg, Texel, the Netherlands; and
    2. Animal Ecology Group, Centre for Ecological and Evolutionary Studies (CEES), University of Groningen, PO Box 14, 9750 AA Haren, the Netherlands
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  • RUDI DRENT

    1. Animal Ecology Group, Centre for Ecological and Evolutionary Studies (CEES), University of Groningen, PO Box 14, 9750 AA Haren, the Netherlands
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j.vangils@nioo.knaw.nl

Summary

  • 1Rate-maximizing foragers that only divide their time between searching and handling prey should, according to the classical contingency model (CM), only select those prey whose energy content per unit handling time (i.e. profitability) exceeds or equals long-term average energy intake rate.
  • 2However, if digestively constrained foragers were to follow this so-called ‘zero-one rule’, they would need to take digestive breaks and their energy intake over total time would not be maximized. They should, according to the digestive rate model (DRM), also consider the rate at which a prey type is digested (i.e. digestive quality), such that time lost to digestive breaks is minimized.
  • 3In three different contexts, we tested these competing models in a mollusc-eating shorebird, the red knot (Calidris canutus), that is often digestively constrained due to its habit of ingesting its bulky prey whole. Measurements on gizzard size (using ultrasonography) and prey-characteristics confirmed that in each test the birds were digestively bottlenecked and should thus follow the DRM in order to maximize long-term energy intake.
  • 4In the first experiment, knots were offered a choice between two fully exposed prey, and tended to select prey by the criterion of digestive quality rather than profitability.
  • 5In the second experiment, knots were offered two buried prey types and preferred the highest quality prey to the most profitable prey.
  • 6In the wild, knots mainly fed on high quality Mya and largely ignored poor quality, but equally profitable, Cerastoderma.
  • 7Thus, each test verified the predictions of the DRM and rejected those of the CM. Given that many species face digestion constraints, we expect that the DRM is likely to explain diet composition in many more studies.

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