Evolutionary origin, proximate causal organization and signal value of the whistle-shake-display of male shelducks (Tadorna tadorna)

This paper focusses on the phenomenon of emancipation in the Whistle-shake, an epigamic display of male shelducks. A comparison with other motor patterns and vocalizations indicates, that the Whistle-shake is a combination of the comfort movement Body-shake, with integrated elements of another display: Bill-tipping with trill. Between Whistle-shake and Body-shake almost no differences exist in total duration, duration of the sub-units they have in common, and in the number of shaking elements.

Artificial rain experiments revealed that the Whistle-shake is not emancipated from the original causal factor, typically influencing the present occurrence of the Body-shake: All males predominantly produced Whistle-shakes when sprayed with water. In addition social factors influence the occurrence of the Whistle-shake. The percentage of shakes consisting of a Whistle-shake was higher in dominant than in subdominant birds. Furthermore, by confronting territorial pairs in spring with either a whistle-shaking or a body-shaking male or an empty neighbouring cage we found that the whistle-shaking male released more Whistle-shakes in the test-males than the body-shaking male. In contrast the test-females, which predominantly produced Body-shakes, did not show any changes in their shaking behaviour in response to the different shaking stimuli presented. Therefore it seems likely that the Whistle-shake and the Body-shake share causal comfort factors but differ in their sensitivity for social stimuli. Context analyses demonstrate that the Whistle-shake shows a sequential association with aggression in dominant males and with escape behaviour in subdominant males. Therefore, the occurrence of the Whistle-shake is not restricted to a specific balance between the activation of motivational systems for aggression and fear.

The dual function of the Whistle-shake (comfort and social function) is discussed.