Wing length as a predictor of body size in the Willow Tit Poecile montanus

Abstract

In a subalpine mixed forest in central Norway, I measured the wing length, tail length and tarsus length and recorded the body mass of 48 male and 43 female first-year Willow Tits Poecile montanus. Some of these were retrapped in the following years and measured again after they had moulted. The wing length of first-year birds was significantly correlated (Pearson) with tail length (males: p<0.01; females: p<0.05) and tarsus length (males: p<0.05; females: p<0.05) but not with body mass. After their postnuptial moult in July-September, i.e. from their first-year to adult stage, the mean wing length of males increased significantly: 0.82 mm from their first to their second year and 0.37 mm from their second to their third year. Also the mean wing length increase in females was marked from first to second year (0.24 mm), but not for the following years. In adult Willow Tits, i.e. two years or older, the wing length was significantly correlated with only tarsus length (p<0.05) in males and tail length (p=0.01) in females. Thus, while wing length apparently is an appropriate measure of body size of first-year Willow Tits, it seems to give an uncertain reflection of the size in adult birds.