Influence of comfort and social stimuli on a comfort movement and a display derived from it

We tested the extent of emancipation of the Whistle-shake display of male shelducks, Tadorna tadorna, from causal factors controlling its presumed evolutionary precursor, the Body-shake, a comfort movement. Both motor patterns show similarities in form and alternate in a yearly rhythm. First, in an artificial rain experiment, we analysed the influence of a stimulus controlling comfort movements. In spring almost exclusively Whistle-shakes were induced even in the absence of social stimuli, while in summer the birds performed predominantly Body-shakes. This is probably related to the seasonal production of sex hormones. Second, we tested the influence of social stimuli by confronting shelducks with displaying and non-displaying males. During moult when males almost exclusively perform Body-shakes we found no influence of social stimulation on the frequency or form of shakes. However, an effect of social stimulation was found in a period when the drakes gradually replace the Body-shake by the Whistle-shake which is after moult in autumn. In this experiment, birds were confronted with either an empty cage (control), artificial rain, a non-displaying or a displaying male conspecific. In relation to the control situation the birds performed more Whistle-shakes when stimulated socially and more Body-shakes when stimulated with artificial rain. The extent to which both shaking patterns share the same causal factors is discussed. Copyright 1998 The Association for the Study of Animal Behaviour.