Social behaviour, pair formation and the behavioural effects of testosterone in the mallard (Anas platyrhynchos).

Despite considerable work on the behaviour of ducks, many questions remain over the behavioural basis of pair formation. Social behaviour was studied here in individually marked captive mallards of wild stock. Particular attention was paid to the roles of 'social display' and of male aggressive behaviour in pair formation. In the autumn social display was more frequent in a group of unpaired birds than in a group comprising paired individuals. At the beginning of the nesting season in spring paired males did not display to their mates, but they did display to unpaired females when their mates had been removed. In many oases a female paired with a drake who had displayed to her. Social displays were also given by a male to a female in the absence of other drakes. These data suggest that social display is more likely to be based on courtship than on, say, hostility between males, except possibly In the case of the 'down- up' display. Dominance hierarchies were formed during the autumnal courtship period, and in the spring the 'territorial' pattern of aggression between paired males changed to a dominance hierarchy when the resident females were replaced by strange females and display behaviour was resumed. The mating preference of a female for one of two males was closely associated with the dominance relationship between the males. Social behaviour was studied in juvenile males injected with testosterone. The treatment facilitated the occurrence of many adult behaviour patterns, although the expression of the behaviour was dependent on the experimental conditions. Social display occurred only if the males were familiar with the females who were present. The displays of the young males were effective in attracting the sexual interest of adult females during the post-breeding period, thus supporting the conclusion that social display is functional in courtship.