Great flexibility in autumn movement patterns of European gadwalls Anas strepera
Andrea Gehrold, Hans-Günther Bauer, Wolfgang Fiedler, Martin Wikelski
The annual migration cycle of waterbirds often involves several distinct movement stages, for example within-winter movements or the moult migration during summer, which require a high degree of individual flexibility in migration direction. Here, we investigate whether such flexibility is a common characteristic of waterbird migration by analysing movement behaviour of a dabbling duck, the gadwall Anas strepera, during the little studied, intermediate autumn period. The tracking of individuals via satellite transmitters (n = 7) as well as the ring re-encounter analysis of three European gadwall populations (Germany, England, Russia) revealed that autumn movements were multidirectional. Furthermore, the comparison with winter re-encounters suggested that autumn movements were partly independent of the movements towards subsequently used south to southwestern wintering areas. Some individuals even travelled long distances north- or eastwards. Accordingly, some autumn locations were characterized by a harsh climate, thus serving as temporary staging sites but necessitating further movements when wetlands freeze during winter. The occurrence of such detours or reversals of migration was confirmed by the transmitter data. Inter-individual variability in distance and direction of autumn movements was found for both sexes and age-classes indicating that gadwalls, in general, followed flexible movement strategies. Based on the extent of multidirectional autumn movements, we hypothesize important benefits of such flights and suggest that the analysis of year-round movement patterns of individual animals during their distinct life-history stages is essential to understand how they can successfully reproduce and survive.