Factors influencing the onset of post‐nuptial moult in Willow WarblersPhylloscopus trochilus

Abstract

A long‐term study (1975 to 1989) on a population of Willow Warblers showed that in each year the majority of male birds commenced post‐nuptial moult before females. It is suggested that this was due, in part, to an annual rhythm in moult onset and was fine‐tuned, in both sexes to different degrees, during their first summer by other endogenous factors possibly linked to breeding. The majority of males commenced moult in June, regardless of whether replacement or second broods were being reared in July, but females synchronised onset of moult to the independence of their young. Two late breeding females suspended moult until their young were independent. Photoperiod alone probably had no bearing on the onset of moult in the majority of birds, but this factor may influence the rate of moult in late moulting individuals. Suspension of moult before migration probably involves late breeding females, and is discussed speculatively.