KLEEFSTRA R, SMIT C, KRAAN C, AARTS G, DIJK J VAN & JONG M DE (2011) Growing importance of the Dutch Wadden Sea as a moulting area for Common Shelduck Tadorna tadorna. LIMOSA 84 (4): 145-154.

Traditionally most European Shelducks migrate to the German part of the Wadden Sea for their post-nuptial moult of flight feathers. The number of moulting Shelducks in Germany has decreased from an average of 206 000 individuals in 1996- 2002 to 155 000 in 2003-2009. At the same time numbers along the Frisian coast in the Dutch part of the Wadden Sea have increased from on average 15 000 individuals in 1981- 1999 to about 30 000 in 2000-2010, with peak numbers in August 2008 (53 000) and August 2009 (more than 77 000, Fig. 4). The presence of these high numbers of moulting Shelducks initiated a series of aerial surveys (in conjunction with counts in the German part of the Wadden Sea) and surveys from ships, in order to study whether these techniques yield proper results in the Dutch Wadden Sea. In August 2010 aerial surveys and ship surveys revealed very similar numbers of moulting Shelducks in the Dutch part of the Wadden Sea: about 54 000 and almost 51 000 respectively, within the same week (Fig. 2). The counts in August 2011 showed a large difference (less than 32 000 from the plane and more than 51 000 from ships), probably due to changes in duck distribution over the two consecutive days over which the aerial counts had to be conducted for logistic reasons.
Earlier studies have shown that Shelducks in the western part of the Dutch Wadden Sea are highly dependent on Mudshrimps Corophium volutator as a food source. Annual mean densities of Mudshrimps (based on annual surveys of benthic fauna in late summer) and numbers of moulting Shelducks (based on counts on high tide roosts along the Frisian coast) show a positive association (Fig. 6). We suggest that abundant food supplies of Mudshrimps (2650 per m2 in 2009) in combination with low human (boating) activity are important factors that have attracted moulting Shelducks to the area. Hence, it is important to protect the area as such, at least in the moulting period from July to August. In order to monitor the development of Shelduck numbers in the Dutch and German part of the Wadden Sea we recommend to continue simultaneous aerial surveys, in the framework of the Trilateral Monitoring and Assessment Program (TMAP). Additional counts from ships yield useful additional information and appear to yield good results in the Dutch Wadden Sea. ####Common Shelduck Tadorna tadorna

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