Wing moult and movement behaviour of anatids, with focus on the European Gadwall (Anas strepera)

Abstract

This thesis addresses two major events in the annual life cycle of birds: the period of moult which is mandatory to maintain the vital functions of the bird’s plumage; and the period of migration which enables birds to reach areas that provide the most suitable conditions during periods of self-maintenance or reproduction. In particular, I investigated the moulting and movement strategies of anatids to find out (i) how these species (which moult all flight feathers simultaneously) adapt to several weeks of flightlessness; and (ii) how these birds optimise habitat use during other parts of the year (given that suitable wetlands are often restricted in size and distributed patchily).To identify the requirements of species that become flightless during moult and to test whether single species differ in their ability to tolerate a range of environmental conditions, I compared habitat use of Gadwalls (Anas strepera), Red-crested Pochards (Netta rufina), Common Pochards (Aythya ferina), Tufted Ducks (Aythya fuligula) and Coots (Fulica atra) at one of the most frequented inland moulting sites in Europe (chapter 1). I found that habitat use of both food specialists and food generalists was related to the water’s nutrient content suggesting an active choice of suitable food sources. In addition, I found that the sensitivity to increasing water depths and to human-induced disturbance could be linked to foraging mode and the species’ attachment to shallow water zones close to the shore. However, the choice of a familiar moulting location seems to provide one strategy to mitigate the impact of suboptimal conditions. Such an effect was observed in tagged Gadwalls which showed a clear preference for moulting locations they had sampled in a previous year.With regard to moult-related physiological adaptations, it has often been postulated that the loss of body weight represents a special adaptation to the flightless stage. However, by analysing weight dynamics of moulting Gadwalls over nine study years, I provide clear evidence that moulting waterbirds can maintain body weight during wing moult (chapter 2). The observed variation in year-specific weight dynamics rather suggested that environmental conditions determine the ability of waterbirds to maintain body condition during moult. Furthermore, I could show that previous reproductive investment may affect the body condition of moulting female Gadwalls – indicating a carry-over effect between the breeding and the moulting season.Similarly, carry-over effects may occur if individuals enter the wintering season in poor condition. Post-moulting birds should thus aim to refuel and accumulate body stores during the intermediate autumn period. However, it appears that habitat specialists, like waterbirds, may not necessarily find profitable feeding spots en route. Indeed, I observed multidirectional or even reversed movement directions in some Gadwalls tracked via satellite transmitters after departure from the moulting site (chapter 3). The fact that ducks moving north or east during autumn located in areas inappropriate for wintering (due to long freezing periods) indicated that such autumn movements may represent a discrete form of migration. This hypothesis was corroborated by the ring recovery analysis of three different European Gadwall populations (Germany, England, Russia), which revealed that both sexes and age classes (inexperienced juveniles and experienced adults) may perform autumn movements that are partly independent of the subsequent migration towards final wintering grounds.In summary, this thesis provides new insights into the habitat use of exceptional habitat specialists (i.e. waterbirds) that depend on a patchy and restricted resource. On the local scale, these birds seem to rely on a rapid sampling of environmental conditions – but probably also on prior experience – to select suitable habitats. On the larger spatial scale, flexible movement strategies may enable waterbirds to discover and exploit suitable seasonal habitats. These findings suggest that both the maintenance of existing key sites and the creation of additional protected zones provide important management tools to enable the conservation of waterbirds throughout their annual life cycle.