Strong winters may increase mortality in avian species, with potentially severe consequences for populations. A period of 10 cold days during an otherwise mild winter occurred in the Wadden Sea during February 2012, causing a mass mortality of more than 1,100 Eurasian Oystercatchers (Haematopus ostralegus). This study investigated whether mortality was highest in certain bird-classes (sex, age, diet specialization). By comparing dead with live individuals captured before and after the cold spell, significantly more immature Eurasian Oystercatchers were found dead (> 60%) than expected. Sex ratio did not differ significantly between dead and live individuals, the proportion of males being higher in both dead and live birds. All the dead individuals collected showed significantly smaller body masses (overall mean: 324 g) than live ones (overall mean: 457 g). Few individuals showed internal lesions or anatomical abnormalities, and no influenza virus was found. Dead Eurasian Oystercatchers had completely empty stomachs in 25% and 37% of the individuals in both study areas, Büsum and Amrum, respectively; all other individuals showing only few prey remains. The most frequent prey items were common cockles (Cerastoderma edule). Based on bill shapes, most of the dead Eurasian Oystercatchers were bivalve specialists, and fewer than expected worm eaters died. From band recoveries (n = 19), many of the dead individuals originated from Scandinavia (37%). The main reason for the high mortality appeared to be low food availability and quality before the cold spell combined with sudden low temperatures.
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Vol. 37 • No. 3