Variations in wing‐lengths of willow warblers in relation to age, sex and season

Abstract

The wing-lengths of adult and first-autumn Willow Warblers were analysed using same-month retrap. to examine measurement consistency between and within ringers. Recoveries in subsequent years were used to investigate growth increments in juvenile birds following the year of hatching and lo compare aduit wing-lengths in later years. 95% of adults and 90% of first-autumn birdscould be sexed on wing-length alone. First-autumn birds (aged using post-juvenile moult, to ensure wing-growth was complete) showed an average increase of 3% in wing-length when recaptured in their first spring. Adult females showed reasonable consistancy in mean wing-lengths between seasons and years, but adult males showed an increased mean in second spring. It is suggested that Willow Warblers do not further increase wing-length (after initial primary moult) with subsequent increases in age. The possibility that a proportion of first-autumn birds do not undergo primary moult in their first winter is discussed speculatively. © 1983 Taylor & Francis Group, LLC.