Reverse migration as a cause of vagrancy

Capsule: Reverse migration in autumn does not occur to the same degree in all species of migrants, but is related to migratory direction. Aims: To identify factors determining degree of reverse migration and specifically to test whether it occurs in long-distance migrant species irrespective of their standard (normal) migration direction. Methods: Multiple regression analysis on the number of individuals occurring as reverse migrants observed in northwest European countries. Results: The number of reverse migrants observed in northwest European countries is strongly correlated with standard migratory direction, estimated population size and detectability, but an effect of the distance travelled from the breeding areas is not supported. The pattern holds true for subsamples of the data set, including British and Irish records or Scandinavian records only, and when controlling for phylogeny. Conclusion: Birds that migrate eastward in autumn from their breeding grounds, mostly in eastern Europe, are more likely to consistently reverse migrate than those species migrating southward mostly from southern Europe.