Exploring Contrasting Trends of Migratory Waterbirds in the Wadden Sea 2009

Wadden Sea Ecosystem No. 27 - 2009

Citation: Ens, B. J., Blew, J., Roomen van, M.W.J., Turnhout van, C.A.M. , 2009. Exploring contrasting trends of migratory waterbirds in the Wadden Sea. Wadden Sea Ecosystem No. 27. Common Wadden Sea Secretariat, Trilateral Monitoring and Assessment Group, Joint Monitoring Group of Migratory Birds in the Wadden Sea, Wilhelmshaven, Germany.


Systematic counts of waterbirds have been conducted since the 1960s in the various countries of the international Wadden Sea. Recently, a lot of progress has been made with regard to data storage and data analysis, allowing a meaningful analysis and comparison of trends in the various regions of the Wadden Sea; for details on methods and trends see Blew & Südbeck, 2005; Blew et al., 2007. The JMMB has carefully calculated and further described the waterbird trends (Blew & Südbeck, 2005). An alarming number of trends is negative. An assessment of these negative trends has suggested several potential causes, including: changes in the breeding or overwintering areas, changes in the food supply in the Wadden Sea, changes in the size of Wadden Sea population and/or the proportion of biogeographical population. However, these assessments were of a purely descriptive nature. The habitat use of barnacle and brent goose has been described by Koffijberg & Günther (2005), effects of hunting regimes on curlew by Laursen (2005) and a particular assessment of shellfish-eating birds has been conducted Scheiffarth & Frank (2005). The quality of hightide roosting sites and potential impacts on these has been described by Koffijberg et al. (2003).

Apart from many trends being negative, some of the trends show remarkable differences between countries. For instance, bar-tailed godwit increases in The Netherlands, shows a stable development in Lower Saxony, but decreases in Schleswig-Holstein and Denmark. Similarly, dunlin increases in The Netherlands, shows a stable development in Lower Saxony and Denmark, but decreases in Schleswig-Holstein. In contrast, oystercatcher declines nearly everywhere, except for Denmark, where it increases. Both from a conservation and management perspective, and from a scientific perspective, it is important to understand the cause, or causes, of these different trends. This led to the current project. The goal is to identify a more or less complete set of hypotheses and name explanatory variables for these hypotheses. Collection of the variables most likely to have an explanatory value and a statistical analysis should help to build understanding of some of the forces behind the different trends. Consequently, results should help to point at or advise on management options and to further focus our attention and efforts for improved data collection and management in the international Wadden Sea.