Abstract

In order to detect seasonal acclimatisation on body mass and fat reserves, patterns in mean monthly dawn body mass and body fat were investigated in a population of Zebra Finches, Taeniopygia guttata, for 17 months in the extreme south-eastern part of their distribution. Mass and fat were lowest in the cold (non-breeding) months and highest in the warmer (breeding) months, resulting in a maximum annual amplitude in body mass of 8% in males and 14% in females. Both sexes increased in body mass and fat at the onset of the breeding season, but there were no significant sex differences. The pattern of winter 'slimming' and summer 'fattening' in Zebra Finches stands in striking contrast to that of small, non-migrant, Northern Hemisphere passerines that make substantial mass increases in winter, rather than summer. However, our findings are consistent with studies on laboratory Zebra Finches. Winter mass loss could be an adaptive response to reduce energetic costs, including those of flight, and it could reduce predation risk associated with frequent travelling in search of dispersed winter food. In a multiple regression analysis, day length, minimum ambient temperature and breeding activity were significant predictors of body mass in males, but not in females. Breeding activity was the most significant predictor, but was positively correlated with day length and temperature. Competitive mating may be a factor in regulating increases in male mass during the breeding season, but further investigations are needed.