The food ecology of the Spoonbill. Platalea leucorodia

In the past the spoonbill Platalea leucorodia used to breed in the Netherlands in far larger numbers than it does today. Recently the cause of the drastic fall in numbers was pollution of the environment with chlorinated hydrocarbons (Rooth and Jonkers,1972) during the years 1963 to 1970 (Fig. 1) Since 1970 the population has been recovering and numbers are increasing. Especially on the Waddensea-islands the increase in numbers of breeding pairs is striking. At several places new colonies have been established. At the moment the spoonbill are breeding in seven colonies all situated in the north-western part of the Netherlands (Fig. 2). Nevertheless, the total number is still much lower than at the turn of the century, or even shortly after the second world war. During the last thirty years the mainland of North-Holland, which forms the the most important foraging area during the breeding season, changed drastically. About 25% of the suitable foraging-areas with shallow waters and small ditches have vanished, owing to changes in agricultural management and urbanization. One of the changes is the artificial manipulation of the water-level. Due to this, many shallow ditches with flat beds and edges sloping gradually, have been replaced by straight deep ditches with steep slopes. In these ditches a strict schedule of a low winter-level (10 to 15 cm) and a steep increase to a high summer-level (20 to 40 cm.) is maintained. Other measures are the reduction of the number of sluices used for water-management and the special measures taken to prevent salt water from penetrating into the mainland. This (for instance) makes it impossible for the migrating three-spined stickleback Gasterosteus aculeatus to enter the fresh water in early spring and causes changes in the stickleback population (Wooton, 1976; Baggerman, 1957; Mullem and van der Vlugt ,1964). Fortunately the foraging-areas on the tidal flats of North-Holland and Texel remained intact and there are no indications at hand of big changes in the prey population (c.q. Crangon crangon).