On the social behaviour of Gadwall (Anas strepera): Displays, pair bonds and effects of testosterone injections.

The thesis provides a description of activities in captive Gadwall, emphasising male epigamic display movements and calls. Analysis was based upon film, video-and audio-tape recordings. Quantitative measures were obtained for the spatial and temporal relations between mates and between members of different age and rearing groups within a flock of 10 to 15 free-ranging, wing-clipped birds. Spatial relations and the activities of each individual were recorded every five minutes on 40 days during nearly two years, and the data were subjected to computer analysis. A relatively close relationship was not only found between mates, but also between any birds of the same group. At different times of the year, different patterns of organisation prevailed. Proximity and synchrony of activities in mates were most frequent during the months preceding the breeding season and infrequent during the breeding season. This held for all pairs, including those in which the female failed to lay eggs, suggesting an inherent tendency for the pair bonds to dissolve in summer. Testosterone was injected into 18 subadult, and 8 adult males in eclipse plumage in order to investigate whether this hormone promotes male epigamic and aggressive behaviour in Gadwall as it has been reported to do in some other duck species. The frequencies of communal courtship and pre-copulatory displays, and in some cases of aggressive actions increased dramatically in the experimental birds, but not in the controls. Thus the testosterone injected birds showed significantly higher frequencies of the recorded activities. Response varied strongly between different males and different tests. It is suggested that this variability is---at least partly---due to the birds influencing each other's behaviour. Temporal aspects of the experiments such as response latency and long-term effects of the treatment are discussed. The injected testosterone advanced voice breaking in the subadult drakes.