The biannual primary moult of Willow Warblers Phylloscopus trochilus in Europe and Africa

The Willow Warbler Phylloscopus trochilus is one of the few bird species that undergoes two primary moults a year, a post-nuptial moult in the breeding area and a moult in the wintering area. Primary-moult data for Willow Warblers from Finland, Sweden, Britain, the Netherlands, Belgium. Guinea-Bissau, Uganda, Kenya, Malawi, Zambia, Zimbabwe, Botswana and South Africa are analysed. The parameters of primary moult (mean starting date, standard deviation of starting date, and duration) are estimated using the techniques of Underhill & Zucchini (T.988 Ibis 130: 358–372) and Underhill, Zucchini & Summers (1990 Ibis 132: 118-12 3). The scheduling of moult in relation to theother main components of the annual cycle, breeding and migration, is considered. The mean durations of post-nuptial moult for P. t. trochilus and P. t. acredula are 36.5 and 38.3 days, respectively; the start and termination of moult for P. t. trochilus are about 3.5 days later for each degree of latitude northwards, and the start and termination of moult for P. t. acredula, are about 10 days later than that of the most northerly populations of P. t. trochilus studied. Females start their postnuptial moult about 10 days later than males. Southward migration commences as soon as post-nuptial moult is complete. There is an increasing constraint on the timing of breeding and post-nuptial moult events at higher latitudes, leading to overlap between them. The duration of pre-nuptial moult is longer than that of post-nuptial moult, and is completed shortly prior to northward migration.