A fraction of White-throated Dippers Cinclus cinclus breeding in Scandinavia migrate south over the Baltic Sea during their short-distance migration to the winter quarters. The aim of our study was to investigate patterns of body mass changes between arrival to and departure from the wintering grounds in northern Poland. From a cost-benefit perspective, according to optimal body mass theory, we expected that fuel loading should occur as late as possible before spring migration, to reduce costs associated with high fuel loads. We were interested if sexes (in view of the larger body size of males) and age classes (in view of the less experienced young individuals) differ in the duration of the fueling period and in rates of increase of body mass prior to spring migration. To address these questions, we used morphometric data from 520 Dippers trapped over 16 wintering seasons (1989/1990–2004/2005), including birds trapped more than once (881 captures in total). Body mass was stable between late October and mid February. The period of intensive body mass increase lasted for about one month directly before departure in all Dippers and started in the second half of February. The rate of increase depended on date in a non-linear fashion: the later the date, the higher the rate, with a tendency to stabilize in the last days of March. In late March, the estimated mean body mass was 20–21% higher in males and 18% higher in females compared to the mean body mass during winter. Immature males increased body mass faster than adults (both sexes) and immature females and reached higher body mass prior to departure (increase up to 36%). Fuel loads are not only sufficient to complete the 100–300 km long migration over the Baltic Sea, but probably also to reach breeding destinations in the Scandinavian Mountains, c. 600 km north from the wintering range in northern Poland.
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Vol. 106 • No. 2