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Estimating variation among individuals in migration direction


  • Kasper Thorup,

  • Jørgen Rabøl,

  • Birgit Erni

K. Thorup (correspondence), Zoological Museum, University of Copenhagen, Universitetsparken 15, DK-2100 Cph. Ø, Denmark. E-mail: – J. Rabøl, Institute of Biology, University of Copenhagen, Universitetsparken 15, DK-2100 Cph. Ø, Denmark. – B. Erni, Statistical Sciences, University of Cape Town, Rondebosch 7701, South Africa.


Among-individual variation in mean migration directions is the basis of evolution of new migration routes and has important consequences for our understanding of the migratory orientation system. This variation in migration directions is also of interest for modelling of migratory flight paths. In test series with caged migrants, the variation among individuals is generally much smaller than the variation within individuals making the variation among individuals difficult to detect with small sample sizes. No methods exist for estimating among-individual variance for directional data. We therefore used simulations to estimate the variation among individual mean migration directions. Among-individual variation was found to be present in at least half of the 34 series analysed. In the 21 series with first-time migrants, our estimates of the variation among individuals ranged from r=0.20 (mean vector length) to >0.99, with median 0.93 and 86% less than 1. We found slightly more variation among individuals in displaced birds and in experiments with manipulated cues. Test series with experienced migrants, presumably having varying goal areas, showed similar estimates (median 0.94 and 77% less than 1).

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