ANNUAL AND DAILY VARIATION IN BODY-MASS AND FAT OF STARLINGS STURNUS-VULGARIS

Seasonal and daily variation in body mass and fat was monitored in Starlings Sturnus vulgaris and related to the different phases of the annual cycle, to changes in temperature and daylength, and to Lehikoinen's (1987) models for body mass regulation of wintering birds. Lean mass and fat were estimated using total body electrical conductivity (TOBEC) and a combined fatscore index for the abdominal and furcular visible fat-depots. With the fatscore, lean mass and fat were estimated better (R2 = 0.89 and 0.92, respectively) than with Walsberg's (1988) interspecific TOBEC (R2 = 0.56 and 0.75), or our intraspecific Starling TOBEC (R2 = 0.56 and 0.82). Lean mass for both sexes decreased over the breeding season, increased again during the first half of moult, and remained constant for the rest of the year (males 75 g and females 71 g). Seasonal changes in reserves and absolute amounts of fat were similar in males and females (8 g in winter and 2 g during moult). In early spring, female lean mass and fat increased by 7 g and 4 g, respectively, so that early in the breeding season females were 5 g heavier than males. Outside the breeding season, females were 4 g lighter than males. Evening body mass and fat were negatively correlated with the mean temperature of the preceding two to four weeks. During the whole year, nightly mass loss fluctuated between 5 and 6 g. Of the two conditions of the ''true winter fattening model'' (Lehikoinen 1987), namely an increase of dawn body mass and of the amplitude of the daily mass cycle, only the first one was supported.