Fatness, Sex and Dominance; Seasonal and Daily Body Mass Changes in Willow Tits

Abstract

Seasonal and daily body mass variation of flock-living Willow Tits Parus montanus was studied in northern Finland during two winters to find out whether or not social dominance affects the size of birds' internal energy reserves. The daily minimum mass increased during autumn, peaked in midwinter and decreased in spring. These changes, however, were small and not rank-dependent. Daily mass fluctuation was much greater than seasonal variation. On average the birds gained 0.82 g (7.1%) in mass during the day. At dawn dominants and subordinates did not have significantly different amounts of reserves either in winter or in spring. In both seasons dominants gained more mass during the day than subordinates. At dusk, dominants were heavier than subordinates in winter but not in spring. When the competitive advantage of dominance was removed by food supplementation, subordinates gained more mass and tended to be heavier in the evening than when only natural food was available. Additional food affected neither daily acquisition nor evening masses of dominants. In intrasexual comparisons, rank-related differences were found neither in fatness nor in seasonal or daily mass acquisition. Nevertheless, relying on the results of the feeding experiment we conclude that dominant, mainly male, Willow Tits take advantage of their priority in resource access and acquire more internal energy reserves than subordinates which are mainly females. Consequently, fasting tolerance may be rank-dependent causing differences in winter survival between dominance categories.