Fluctuating asymmetry and autumn migration in Willow Warbler Phylloscopus trochilus L.

Abstract

Fluctuating asymmetry (FA) reflects the ability of animals to cope with environmental and genetic stress. Studies made so far have not yet focused and discussed FA's possible effects on migrating birds. In migrating passerines, FA may reflect an individual's overall condition and dominance during the moulting and pre-migratory period . We studied how FA correlates with the timing of migration in one common migratory species, the Willow Warbler (Phylloscopus trochilus L.) . In our study, adult females that migrated earlier have more asymmetrical outermost tail feathers than those migrating later. In males, there was no significant correlation between FA and timing of migration. Adult females moult later than males and thus may suffer from time constraints in their moult before migration. Competition for food resources in contemporary migrating females may explain why more asymmetrical, probably low quality females, may have to migrate first.