Rank-dependent winter fattening in the Willow Tit Parus montanus

Abstract

The body mass in tits increases in autumn towards awinter peak in December-January and then declines towards spring . I studied the winter fattening in the Willow Tit Parus montanus at two different sites in central Norway. Body mass data were collected in the early morning, late evening and partly at noon. To make the data comparable, they were transformed to body mass indexes. In rank-structured singlespecies flocks or aggregations of tits the body mass of each individual was compared with each one of the other members, using paired t-test (2-tailed) to control for seasonal and diurnal body mass fluctuations and impact of ambient temperature. Subordinate Willow Tits carried on the whole significantly more body fat than dominants throughout the day. Dominants do not utilize their actual capacity for fat storage, suggesting that fat storage is costly . Large fat deposits obviously reduce the risk of starvation, but whether the risk of predation at the same time is reduced because of constricted manoeuvrability is dubious. Since the dominants have priority to food resources, the subordinates are forced to carry more fat as an insurance against starvation . No consistent rank-dependent differences were found with regard to the diurnal body mass cycle.