Tail length of wagtails (Genus Motacilla):

Correlates w ith body size, habitat type and behaviour

It was hypothesized that the noisy environment along rivers calls for alternative ways of communication. In birds living near rivers, a long tail and wagging may serve as an optical communication system and thus may Support vocal communication. The two species of wagtails (M. cinerea and M. clara) which live near rivers have indeed longer tails than expected from interspecific allometry across other species of the genus Motacilla. However, these two species are closely related. Thus, there are no degrees of freedom for convincing conclusions. Furthermore, there are other hypotheses which may explain the long tail, in particular sexual selection and aerodynamics. We show that the latter may also explain the long tail of certain wagtail species.