KLEEFSTRA R & SCHEKKERMAN H (2019) Curlew Sandpipers Calidris ferruginea on high tide roosts near Westhoek, central part of the Dutch Wadden Sea. LIMOSA 92 (2): 65-73.

The Curlew Sandpiper Calidris ferruginea is a scarce species in the Dutch Wadden Sea, with short migration peaks during spring (May) and autumn (July-September). Due to these short peaks, the population cannot easily be assessed accurately using the regular monthly counts of waders on high tide roosts. In order to get a better estimate of the migrating numbers of Curlew Sandpipers in the Dutch Wadden Sea during autumn migration, we conducted weekly counts on high tide roosts along the Frisian Wadden coast, over a stretch of 11 km from Koehool to Zwarte Haan, in the period July 2001-September 2018. The most important roosts are the salt marshes of Westhoek and Zwarte Haan (Fig. 1). In 2001-10, counts were largely conducted in the period mid-July to mid-August, whereas in the years 2013-18, counts were largely performed from the beginning of July to the beginning of September. No counts were conducted in 2011 and 2012. The maximum number of Curlew Sandpipers during the first peak of autumn migration (end of July-beginning of August) were between 2110-8000 individuals (Fig. 2). The maximum numbers of Curlew Sandpipers during the second peak of autumn migration (end of August-early September, years 2013-18) were always lower than the first migration peak (740-2980 birds; Fig. 2), with two outliers in 2013 and 2016. These two outliers were years with a high breeding success, arguably due to Lemming peak years, likely with high numbers of migrating juveniles. The trends of our weekly counts clearly do not match with the regular monthly counts for the months July, August and September (Fig. 4). In fact, the average number of Curlew Sandpipers counted during these weekly counts are higher in all three months (Tab. 1). This difference can be explained by the fact that during monthly counts the short migration peak easily can be missed. No juveniles were counted in July and early August, while from mid-August to mid-September the proportion of young birds was 50% to almost 100%. The high tide roosts near Westhoek, in the central part of the Dutch Wadden Sea, have become important stopover sites for Curlew Sandpipers. It is one of the species, just like Common Shelduck Tadorna tadorna, Pied Avocet Recurvirostra avosetta and Common Redshank Tringa totanus, of which numbers declined in the German Wadden Sea in summer and autumn, while numbers in the (central part of) the Dutch Wadden Sea have increased. High densities of Mud Shrimps Corophium volutator can be a potential reason for this development.

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