Social dominance and energy reserves in flocks of Willow Tits

Theoretical studies have suggested that birds in winter should carry higher energy reserves when food supply is lower, or less predictable, in order to maximize their probability of survival until the end of winter. In the Willow Tit Parus montanus, a passerine wintering in dominance-structured flocks, subdominant birds were found to carry higher energy reserves than dominant birds (Ekman and Lilliendahl 1993, Behav. Ecol. 4: 232-238). Since food supply is probably lower (and less predictable), for subdominant birds, this seemed in agreement with theoretical results. However, we analysed the effect of social dominance on energy reserves using data from another Willow Tit study (Hogstad 1987, Auk 104: 333-336), and found that in this study dominant birds carried the highest energy reserves. In Willow Tits, social dominance is known to affect predation risk during foraging. Using a simple analytical model, we show that when social dominance affects predation risk while foraging, but not food acquisition rate, the optimal level of energy reserves is higher for dominant than for subdominant birds. When social dominance affects both food acquisition rate and predation risk, its effect on the optimal level of energy reserves depends on the relative importance of these two factors. Thus variation in the effect of social dominance on food acquisition rate and predation risk may explain the difference between the two studies compared.