Is variation in orientation related to fluctuating asymmetry in migratory passerines?

Abstract

Fluctuating asymmetry is considered an indirect measure of phenotypic quality and can directly impose high costs in terms of aerodynamics in flying birds. The accuracy of orientation during migration in small passerines is relevant for survival and reproduction, but patterns of individual variation in preferred directions have received little attention. I tested the hypothesis that symmetrical individuals are able to more accurately orient in the correct direction than asymmetrical individuals. Wing, tail and tarsus asymmetry were measured in blackcaps (Sylvia atricapilla), redstarts (Phoenicurus phoenicurus) and robins (Erithacus rubecula), and the birds were tested for orientation in Emlen funnels, at two stopover sites in N Europe. I then matched individuals for test date, species, sex and age, and assigned them to either of two groups: symmetrical or asymmetrical. The groups were not different in any of the orientation parameters. The lack of effect of asymmetry on orientation may imply that: (i) choices of different direction reflected different, but equally effective migratory strategies; (ii) symmetrical birds could afford longer stopovers while asymmetrical ones be less motivated to migrate; (iii) differences in orientation behaviour would have reflected different quality only under novel or anomalous conditions or (iv) fluctuating asymmetry is not a useful measure of quality.