| Abstrakt: |
Despite major implications, moult remains understudied compared with other major annual cycle phenomena such as breeding and migration. This is certainly true for Eurasian Spoonbills Platalea l. leucorodia for which even the primary moult has remained poorly described. Using digital photography, we studied primary moult of Spoonbills in the eastern Dutch Wadden Sea from July to October 2021. From the images that were sufficiently clear to quantify primary moult we obtained a total of 168 observations for immatures and 790 for adults. We aged spoonbills by the extent of black on the primary tips and scored their abdominal profile as an index of body mass. Progression of the descendent moult showed a distinct sigmoid pattern, implicating population turnover at the start and end of the study period. Also, few individuals completed moult in the study area. Therefore, we applied Underhill-Zucchini models to actively moulting individuals. As expected, immatures were estimated to initiate moult over a month earlier (11 June) than adults (20 July). Surprisingly, immature spoonbills moulted at a higher rate, which would allow immatures to complete primary moult in 131 days without suspension compared with 170 days in adults. As a fast moult may reduce feather quality, we speculate that the black wing tips of immature spoonbills may compensate for lower feather quality. Birds with low abdominal profile scores were observed only in July and August, but fat birds were seen throughout the study period. The incidence of suspended moult sharply increased through September in both age groups. By late September 30–40% of birds had suspended moult and abdominal profile scores peaked. This means that immature and adult spoonbills appear to synchronize the suspension of moult before migration, despite marked age differences in the onset and rate of primary moult. Spoonbills may benefit from synchronized migration by improving navigational accuracy and flight efficiency through collective decision-making and formation flights. [ABSTRACT FROM AUTHOR] |
