Veranderingen van ruiplaatsen van Knobbelzwanen Cygnus olor in het noordelijke Deltagebied

Changes of moulting sites for Mute Swans Cygnus olor In the northern part of the Dutch Delta The distribution of moulting Mute Swans in the northern part of the Dutch Delta showed considerable changes during 1989-1994. Up to and including 1988, the highest numbers spent their wing-moult on the former estuary Lake Grevelingen, a saltwater lake separated from the sea as a part of the Dutch deltaworks in 1971. On Lake Grevelingen Seeweed Zostera marina was the most principal food of the moulting swans. During the eighties Seeweed disappeared almost completely from Lake Grevelingen. In 1987 the Volkerak-Zoommeer, situated nearby Lake Grevelingen, was closed and transformed to a freshwaterlake. Table 2 shows how during summer 1989 Mute Swans moved from Lake Grevelingen to the Ventjagersplaat, situated in the Haringvliet and to Volkerak-Zoommeer, where moulting Mute Swans concentrated on two sites, the Philipsdam and a second along the coast of Overflakkee with a channel with heavy shiptraffic in between. Summer 1993 and 1994 moulting Mute Swans were monitored. Summer 1993 the sites Ventjagersplaat and Philipsdam held important concentrations. Autumn 1993 on both sites considerable works took place. Summer 1994 at first Mute Swans concentrated along the Overflakkeecoast with 1930 birds at the end of June. Then birds started to move to Ventjagersplaat and to Philipsdam, crossing the channel. Mid-July numbers had climbed to 3470 birds, divided over the three sites. Supposedly at the end of June the carrying capacity of the site along Overflakkee was no longer sufficient to feed the whole concentration.