1. A flock of ten silvereyes kept under regular food deprivation of one hour per day exhibited a dominance hierarchy resulting from aggressive encounters. The weight gain of these birds in six weeks of winter was significantly less than that of the control flock of ten birds kept with uninterrupted food supply. 2. In the experimental flock the omega bird lost weight whereas the two most aggressive birds gained considerably less weight than the others. This was interpreted to mean that both agonistic behaviour and the resultant social organisation have biological significance in the survival of individuals. 3. It is proposed that winter flocking serves a conservation function in exploiting limited food sources and that agonistic behaviour is an integrating factor in the organisation of the flock.