NOORDHUIS R & TULP I (2002) Bewicks Swan's in the IJsselmeer area. Have the good times passed?. LIMOSA 75 (1): 13-24.

N.b. The original version of this paper was published in Limosa 12: 1-18, 1939. Besides Ireland the Zuiderzee in the Netherlands is one of the most important hibernating places of Bewick's Swans (cf. Ten Kate 1930) In the winters of 1936-1937 and 1937-1938 their number fluctuated between 1800 and 3200. The distribution of the swans in the Zuiderzee has changed 5ince part of it was enclosed in 1932. The changes in the distribution are rep resented in the map of Fig. 1. which is based on information, collected from fishermen . farmers, hunters and ornithologists. Since 1935-1936 this information has been checked by censuses by the authors and their collaborators.
      T0 explain the causes of the changes the feeding-habits of Bewick's Swans are described. The animals feed swimming in shallow water (of 0.2-0.6 rn, sometimes up to 1 In depth). They make excavations with their feet (and perhaps with their bills) in the sand and clay bottom. Here they attain the roots of different aquatic plants (at present chiefly rhizoms of Potamogeton pectinatus and Zannichellia pedicelliata; in the former Zuiderzee also rhizams of Zostera marina) by dipping their heads and long necks into the water or by grovelling. The composition of the food was studied by microscopic analysis of faeces. Two factors of distribution can be deduced from the feeding-habits: (1) The distribution of the food-plants, (2) The depth of the water, which may not exceed 1 m. The maps of Fig 3 and 4. representing the distribution of vegetation before and after desalination. when compared with Fig 1, show the great parallelism between the distribution of the swans and of the vegetation in the part of the Zuiderzee south of the line Enkhuizen-Stavoren (Fig. 1). The influence of the depth of the water is not shown in the present Zuiderzee, as the veget8tion does not exceed the 1 m isobath. In the northern part of the Zuiderzee, a third factor, as yet unknown 10 us, may have had same influence. Although Zostera-vegetalion was abundant no swans were found in these parts before the Zuiderzee was shut off.

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