BROUWER GA & TINBERGEN L (2002) Feeding habitats and some of the factors influencing the distribution of Bewick's Swan, Cygnus b. bewickii Yarr., hibernating in the Zuiderzee. LIMOSA 75 (1): 2-12.

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 since part of it was enclosed in 1932. The changes in the distribution are represented 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.
      To 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 m, sometimes up to 1 m 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 pedicellata; in the former Zuiderzee also rhizoms of Zostera marina) by dipping their heads and long necks into the water or by grovelling. The composition of the food was studied by microscopie 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 discribution of vegetation before and alter 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 vegetation does not exceed the 1 m isobath. In the northern part of the Zuiderzee, a third factor, as yet unknown to us, may have had some influence. Although Zostera-vegetation was abundant no swans were found in these parts before the Zuiderzee was shut oft. Bewick's Swan Cygnus bewickii

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