DIJK K VAN (1991) Origin and age composition of moulting Mute Swans Cygnus olor at Lake IJsselmeer, The Netherlands. LIMOSA 64 (2): 41-46.

Lake IJsselmeer is the most important moulting area for non-breeding Mute Swans Cygnus olor in The Netherlands (maximum 4750 birds). The birds concentrate along dikes where they feed on green algae. The main study area was the Houtribdijk (52?37'N.5?25'E) which may hold 5075% of the total population present (fig. I). The aim of this paper is to analyse the origin and age composition of the moulting birds along the Houtribdijk. Most data were collected in 1983 when 488 flightless swans were caught. The fraction of one year old birds was 32-35%. Males were predominantly caught in July, females in August (tab. I). Mean age was 3.0 (fig. 2). Almost onethird (31 %) was at least five years old. Some of these are failed or former breeders as at least eight former breeders were recorded in the moulting flocks. Recoveries (fig. 3) reveal that the moulting swans belong to the Dutch breeding population only (3000-4000 breeding pairs). This is in agreement with the large fraction of moulters which belong to the light morph (41.4%, divided into 33.3% of the 66 and 50.5% of the <;J<;J). Up to 29% of the swans returned to Lake IJsselmeer one year later. There is some exchange between Lake IJsselmeer and moulting areas in Zeeland (Lake Grevelingen), but apparently no exchange between Lake IJsselmeer and moulting sites in the Baltic area. More than half of the ringed birds reported dead (56.2%) were shot (tab. 2). Mute Swans are protected by Dutch law, but many of them (a suppposed 2000 annually) are legally shot, due to presumed damage in agriculture. Most swans die in early spring (fig. 4). After one year 17.2% of the one year old birds and 8.3% of the adult birds were reported dead. After six years 24.2% of the one year old swans and 19.3% of the adults were reported dead.

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