ENS BJ, VAN LEEUWEN M, OOSTERBEEK K, NIENHUIS J & ALLEN AM (2019) Wintering areas of Oystercatchers Haematopus ostralegus breeding in the Netherlands. LIMOSA 92 (2): 74-86.

In recent decades Oystercatchers have declined rapidly in the Netherlands. As part of the Year of the Oystercatcher, organized in 2008 to draw attention to the decline and initiate research into its causes, volunteer ringing groups were stimulated to colour-band Oystercatchers throughout the country. Furthermore, a website was developed (www. wadertrack.nl) where volunteer observers could report observations of colour-ringed individuals and directly gain access to their life histories. In the autumn of 2018 wadertrack contained approx. 200 000 observations of approx. 15 000 individual Oystercatchers. We analyzed connectivity, i.e. the relationship between the breeding and non-breeding (wintering) areas, for adult Oystercatchers, which shows high site fidelity to both breeding and wintering areas. We defined winter as the period between August 1 and January 31 and summer as the period between March 1 and June 30. Oystercatchers breeding in the northern part of the country mainly wintered in the Wadden Sea, whereas Oystercatchers breeding in the south mainly wintered in the Delta area. However, there was no clear-cut demarcation and some Oystercatchers breeding in the Delta wintered in the Wadden Sea and vice versa. Furthermore, the wintering area of a considerable number of birds remained unknown, especially for inland breeders (on average 59%) and less so for coastal breeders (on average 35%). We think these birds winter in the Wadden Sea in areas rarely visited by observers during winter, as well as estuaries along the coast of France and the United Kingdom, where observation intensity may be less or observations are not reported to Wadertrack. Oystercatchers wintering in the Eastern Scheldt (containing the largest area of intertidal mud flats in the Delta) suffer from habitat loss due to erosion of the tidal flats. Oystercatchers wintering in the Wadden Sea are confronted with hand gathering of Cockles Cerastoderma edule, which increased after mechanized cockle fishing was stopped. Though Mussel Mytilus edulis beds returned after overfishing in 1990, they are increasingly mixed with Pacific Oysters Magallana gigas, which continue to increase. As a result of differences in connectivity, Oystercatchers breeding in different parts of the Netherlands will be affected differently depending on whether they over-winter in the Wadden Sea, Delta or outside the Netherlands.

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