SWENNEN C (1970) Bird observations on the IJsselmeer. LIMOSA 43 (1): 1-10.

During the winter 1968-1969 monthly bird counts were made from a vessel on the open water of the northern part of the IJsselmeer (former Zuiderzee). The water is now eutrophic and oligohaline (Clorinity between 0.10 and 0.30 % C1'), the depth is 3% to 5% m (maximally to 7 m). The area of open water is in the northern part 520 km2, the whole lake has about 1350 km2 open water with a depth of more than 3% m. The most numerous birds were the Goosander (Mergus merganser) having been on the average 2600 individuals at a stretch of 10 km (area about 20 km2), the Scaup (Aythya marila) and the Blackheaded Gull (Larus ridibundus) on an average more than 400 individuals per 10 km. An average of more than 10 birds per 10 km was reached by the Goldeneye (Bucephala clangula), the Red-breasted Merganser (Mergus serrator), the Common Gull (Larus canus) and the Little Gull (Larus minutus) (Table 2, 3 and 4). The more shallow coastal strips showed apart from the numerous Blackheaded Gulls a totally different bird population, consisting mainly of Mallard (Anas platyrhynchos), Tufted Duck (Aythya fuligula), Pochard (Aythya ferina), Goldeneye (Bucephala clangula), Mute Swan (Cygnus alar) and Coot (Fulica atra). The similarity between the bird population on the open water of the IJsselmeer and on the open water of the connecting part of the Waddenzee is considerable. However, most species of the coastal part do not occur in the Waddenzee. Before the enclosure of the Zuiderzee (the recent IJsselmeer) the Scaup and the Blackheaded Gull were already numerous on the open water of the Zuiderzee. The Goosander, which is occurring in large quantities in the whole lake at present was at that time, however, numerous only in the vicinity of the mouth of the river IJssel. NIOZ Great Crested Grebe Podiceps cristatus Greater Scaup Aythya marila Tufted Duck Aythya fuligula Common Pochard Aythya ferina Common Goldeneye Bucephala clangula Goosander Mergus merganser Red-breasted Merganser Mergus serrator Great Black-backed Gull Larus marinus European Herring Gull Larus argentatus Common Gull Larus canus canus Black-headed Gull Chroicocephalus ridibundus = Larus ridibundus Little Gull Hydrocoloeus minutus = Larus minutus Mallard Anas platyrhynchos Mute Swan Cygnus olor Eurasian Coot Fulica atra Great Cormorant Phalacrocorax carbo Long-tailed Duck Clangula hyemalis Eurasian Wigeon Anas penelope Black Scoter Melanitta nigra Common Eider Somateria mollissima Greylag Goose Anser anser Common Tern Sterna hirundo Smew Mergellus albellus

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