LEBRET T (1971) Observations of surface-feeding ducks Anatinae in wing moult in tidal habitat in the Biesbosch-Hollands Diep-Haringvliet-area. LIMOSA 44 (1): 29-44.

1. The extensive tidal fresh and brackish water marshes in the Biesbosch Hollands Diep - Haringvliet area are of great importance as a wing moult habitat for surface feeding ducks. The tide has been eliminated in this area in November 1970. 2. Eleven different moulting areas are described. 3. Observations on escape and feeding behaviour are described. Part of the observations were made from a small boat, hidden in the tidal marshes. The ducks in wing moult could be observed undisturbed from this boat both at day time as well as during the night. 4. Numbers of ducks in wing moult are estimated for each marsh within the area. Mallard Anas platyrhynchos and Teal Anas crecca moult in considerable numbers. For other duck species the area is less important as a moulting habitat. Information from local experts such as reserve wardens, game keepers and a decoyman has also been used. The results are shown in table 1. The area as a whole may harbour some 13.00018.000 Mallards and 3100-----3400 Teal in wing moult. As only adult ducks moult their wing feathers, the numbers of moulting ducks may also be calculated, if we know the total numbers of the ducks in the area and the proportion of adults in the population. This proportion can be estimated at 1/3 of the total population at the beginning of August. There has been one simultaneous waterfowl count in the whole area on August 26th 1966. After some correction this count yields a number of 41.000-----47.000 Mallards. The use of the 1/3 ratio suggests that some 13.600-----15.600 Mallards moult their wing feathers in the area. The estimate of 18.000 Mallards in wing moult therefore may be too high. 5. The characteristics of the wing moult habitat are described. The tidal mudflats provide the main food source of the ducks in wing moult. Reeds are the most important cover in eight areas. Salix ozier, Scirpus marsh and Spartina marsh occur each in one case only. The isolation of the area depends mainly on the presence of the tide. The tidal marshes are accesible only with difficulties both by boat and on foot. Moreover they are considered to be dangerous and dirty. 6. A prognosis on the effect of the elimination of the tide is given. The loss of the tide will be especially serious as far as it affects the isolation. After some years new food sources may develop, but sufficient cover is likely to develop in only a few cases. These areas will need special protection.

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