Body Mass Dynamics of Pink-Footed Geese (<i>Anser brachyrhynchus</i>) During Stopover on Autumn Migration in Norway

Body mass accumulation is a widely used measure of waterfowl condition and predictor of fitness. So far, however, post-breeding changes in body mass affecting autumn and winter condition have been largely unexplored. Here, changes in body mass of Pink-footed Geese (Anser brachyrhynchus) were investigated during an autumn stopover in Nord-Trøndelag, Norway. Average juvenile body mass varied from year to year, ranging from 1,976–2,286 g and 2,154–2,404 g for females and males, respectively, and was affected by May temperature on the breeding grounds—a proxy for onset of the breeding season. Average adult body mass did not vary between years. During the stopover, juveniles of both sexes increased their body mass substantially (11.4 ± 2.8 g/day), while adult birds showed sex-specific differences. Adult males took on an average of 6.1 ± 2.4 g/day, whereas adult females showed no increase during the period. One explanation of this might relate to sex-specific behaviors in the preceding brood-rearing period. In addition, arrival from the breeding grounds was earlier for non-breeders compared to family groups. Our findings indicate that annual variation in body mass, as well as body mass changes during an autumn stopover, may differ considerably between age and sex classes of Pink-footed Geese.