Wing-moulting waterbirds maintain body condition under good environmental conditions: a case study of Gadwalls (Anas strepera)

Wing moult is a critical period within the annual cycle of birds, particularly in waterbirds which become completely flightless. The inherent vulnerability to anthropogenic disturbance, predation and decreasing habitat quality often results in remarkable body weight loss. However, moult-related changes in body weight can be explained by two hypotheses: The 'adaptive weight loss' hypothesis suggests that the reduction of body weight is a special adaptation to flightlessness, whereas the 'environmental constraint' hypothesis suggests that weight dynamics mainly depend on local environmental conditions. To assess these two scenarios, weight changes of moulting Gadwalls (Anas strepera) were measured during 9 study years. We also analysed the effect of sex-specific differences in timing of moult and previous reproductive investment. Over all years, flightless males lost on average 3.9 % and females 10.6 % of body weight, yet both sexes recovered weight towards the end of moult. In single years, male weight significantly decreased during only 1 out of 9 and female weight during 3 out of 8 moulting seasons. Only female weight dynamics changed considerably among and within these seasons. In particular, females were significantly lighter when moulting late, a trait that is characteristic of successful breeders. Lower average weight levels of moulting females following breeding seasons of higher reproductive output further highlight the connectivity of these consecutive life history stages. Overall, our data indicate that moult-related changes in body weight result from environmental circumstances, rather than being an adaptation to the flightless stage per se. Appropriate moulting sites should be created and protected because adverse habitat conditions during moult could have direct as well as long-lasting fitness-relevant effects on waterbirds.