FATNESS, SEX AND DOMINANCE - SEASONAL AND DAILY BODY-MASS CHANGES IN WILLOW TITS

Seasonal and daily body mass variation of flock-living Willow Tits Pan ts montanus was studied in northern Finland during two winters to find out whether or not social dominance affects the size of birds' intern al energy reserves. The daily minimum mass increased during autumn, pe aked in midwinter and decreased in spring. These changes, however, wer e small and not rank-dependent. Daily mass fluctuation was much greate r than seasonal variation. On average the birds gained 0.82 g (7.1%) i n mass during the day. At dawn dominants and subordinates did not have significantly different amounts of reserves either in winter or in sp ring. In both seasons dominants gained more mass during the day than s ubordinates. At dusk, dominants were heavier than subordinates in wint er but not in spring. When the competitive advantage of dominance was removed by food supplementation, subordinates gained more mass and ten ded to be heavier in the evening than when only natural food was avail able. Additional food affected neither daily acquisition nor evening m asses of dominants. In intrasexual comparisons, rank-related differenc es were found neither in fatness nor in seasonal or daily mass acquisi tion. Nevertheless, relying on the results of the feeding experiment w e conclude that dominant, mainly male, Willow Tits take advantage of t heir priority in resource access and acquire more internal energy rese rves than subordinates which are mainly females. Consequently, fasting tolerance may be rank-dependent causing differences in winter surviva l between dominance categories.