SOCIAL-DOMINANCE AND ENERGY RESERVES IN FLOCKS OF WILLOW TITS

Theoretical studies have suggested that birds in winter should carry h igher energy reserves when food supply is lower, or less predictable, in order to maximize their probability of survival until the end of wi nter. In the Willow Tit Parus montanus, a passerine wintering in domin ance-structured flocks, subdominant birds were found to carry higher e nergy reserves than dominant birds (Ekman and Lilliendahl 1993, Behav. Ecol. 4: 232-238). Since food supply is probably lower (and less pred ictable), for subdominant birds, this seemed in agreement with theoret ical results. However, we analysed the effect of social dominance on e nergy reserves using data from another Willow Tit study (Hogstad 1987, Auk 104: 333-336), and found that in this study dominant birds carrie d the highest energy reserves. In Willow Tits, social dominance is kno wn to affect predation risk during foraging. Using a simple analytical model, we show that when social dominance affects predation risk whil e foraging, but not food acquisition rate, the optimal level of energy reserves is higher for dominant than for subdominant birds. When soci al dominance affects both food acquisition rate and predation risk, it s effect on the optimal level of energy reserves depends on the relati ve importance of these two factors. Thus variation in the effect of so cial dominance on food acquisition rate and predation risk may explain the difference between the two studies compared.