Habitatkeuze van de Waterral (Rallus aquaticus L.) op het Waddeneiland Vlieland

We have tried to account for the habitat choice of the Water Rail on the Waddensea-island of Vlieland in relation to the relevant ecological environmental factors. We studied the occurrence and distribution of the Water Rail in the years 1976 – 1978, the hydrological situation of the soil, the composition of the vegetation, the soil substrates belonging to it the number and length of the borderlines between the different vegetation types. In the months of March, April and May the nesting areas were to be found for the major part on the border of the dune and the wet to very wet dune valley respectively drift-sand dike and Kroonspolder (drift-sandpolder). The vegetation there consisted chiefly of impenetrable brushwood, shrub and of superannuated, dry grasses and sedges, forming clumps of grass and sods. Later in the breeding season (May – June) the nesting areas shifted to the vegetation of the hygrosere of the wet dunes. The pulli were mostly found on soil substrates they can easily walk on. These soil substrates were at most covered with a layer of fresh or somewhat brackish water not more than some centimeters deep and a vegetation with an open structure in view of possibilities for cover. In the summerseason the breeding birds stayed out in some 13 vegetation types (table 1). The soil substrates of the vegetation types belonging to the hygrosere of the wet dunes consisted of fine sand, blue-black mud, slime, sapropelium or humus. This humus had been formed in the wet parts of the area mostly under anaerobic circumstances. In the winterseason the Water Rails occurred much more scattered. In this period the groundwater-level is notably higher than in summer. The area where Water Rails occurred were border-situations with sharp borderlines between the different vegetation types. The catching results (De Kroon 1983) showed that they were mainly successful if the riddle claptraps (De Kroon 1979) had been placed on these borderlines. There is positive correlation (figure 3) between the number of breeding pairs and the hydrological situation of the soil. Likewise between the number of breeding pairs and the number of borderlines between the various vegetation types (figure 6). The same holds good for the number of breeding pairs and the length of the borderlines between the various vegetation types (figure 7). It appears that these are not the only ecological environment factors influencing the settlement of breeding pairs. The fact that there is little or no vegetation and consequently insufficient chances for taking cover, or a vegetation structure which does not allow the birds to pass through it, could also play a causal part.