SCHEKKERMAN H., P. DE BOER, S. DEUZEMAN, J. POSTMA, E. VAN WINDEN, C. KAMPICHLER, M. VAN ROOMEN & DE WADDENUNIT (2015) A rough estimate of numbers of waterbirds wintering on the open water of the Dutch Wadden Sea. LIMOSA 88 (3): 136-144.

In the winters of 2010/11-2013/14 we conducted ship-based counts of waterbirds in 30 strip-transects in the deeper (sublittoral) parts of the Dutch Wadden Sea during high tide. We corrected the counts for distance dependent detection probability and extrapolated them to estimates of total numbers present in the offshore parts of the Wadden Sea (Tab. 1). Our dataset has its limitations in terms of sample size and an over representation of waters with abundant Mussels Mytilus edulis. This probably explains why our estimate for Common Eider Somateria mollissima is twice as high as the average number obtained by dedicated aerial counts. Although our estimates for most other species, whose occurrence was not clearly associated with sublittoral Mussel beds, are less likely to be biased upward, they should be considered as rough approximations. However, they may form a basis for assessments of changes in bird numbers through comparisons with future surveys. The past 50 years have seen some changes in numbers that far exceed the uncertainty surrounding our estimates, e.g. from c. 40,000 Common Scoters Melanitta nigra in the early 1960s to around 1000 or less, and from c. 1,000 Longtailed Ducks Clangula hyemalis until the 1980s to at most a few hundreds nowadays. Furthermore, our estimates improve insight in the coverage of the regular shore-based high-tide counts in the Wadden Sea in terms of total bird numbers present on both land and water (Tab. 2). While this coverage is low (<50%) for most diving waterbird species (which is why trends for Greater Scaup Aythya marila, Common Eider and scoters Melanitta spp. are based on aerial counts), it appeared to be rather high (70-90%) for most gulls.

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