Is tail wagging in white wagtails, Motacilla alba, an honest signal of vigilance?

The function and adaptive value of tail wagging in many bird species are not yet fully understood. Possible functions are flushing insects, displaying submission and signalling an individual's state of alertness to predators. To distinguish between these, I observed feeding and preening white wagtails in August–September 2004 near Husum, Germany. I counted vigilance events (head-up and left–right scans), tail wags and pecks during feeding. Adults and juveniles did not differ in the frequency of tail wagging. Feeding wagtails wagged more often than preening wagtails and, among feeding wagtails, wagging was positively correlated with scanning and negatively correlated with pecking. In preening wagtails wagging was positively correlated with left–right scans. The prey-flushing hypothesis was partially supported, but seems unlikely since wagging occurred during preening and was negatively correlated with pecking. As wagging did not differ between adults and juveniles it is unlikely to signal submission. It may be an honest signal of vigilance during both feeding and preening; this would explain the negative correlation between wagging and pecking because wagtails adopt a nonvigilant posture during pecking.