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Keywords:

  • area-restricted search;
  • correlated random walk;
  • Cygnus columbianus bewickii;
  • depletion;
  • Potamogeton pectinatus

Summary

  • 1
    Tundra swans forage on below-ground pondweed tubers that are heterogeneously distributed in space. The swans have no visual cues to delineate patches. It was tested whether swans employ an area-restricted search tactic. Theory predicts that swans should alternate between an intensive (low-speed, sinuous) search mode in high tuber density areas and an extensive (high-speed, directed) search mode between these areas.
  • 2
    A quantitative analysis of movement paths recorded over short time frames (15 min) revealed that the sequential step lengths were strongly autocorrelated. After partitioning the data in low-speed paths and high-speed paths, this autocorrelation was very much reduced.
  • 3
    Movement paths with low speed were non-directional and could well be described as random walks. In contrast, high-speed paths were directed forward, and were better described as correlated (i.e. directional) random walks.
  • 4
    Movement paths recorded over longer time frames (1–4 h) provided empirical evidence that an alternation of low-speed, sinuous and high-speed, directed searches occurred.
  • 5
    There was a spatial autocorrelation in tuber biomass density, being significantly positively correlated until c. 10 m distance. The scale of the food clump size and step length of high-speed paths matched, suggesting that they were causally linked.
  • 6
    Computer simulations confirmed that swans using the observed search tactic achieved a higher energy gain than swans using only an intensive search mode, provided that the tuber biomass density occurred in clumps. They also achieved a higher gain than swans that alternated between intensive and extensive search mode, but always moved in a random direction.