Bewick’s Swans Cygnus columbianus bewickii in the Netherlands: numbers, distribution and food choice during the wintering season

Abstract

Numbers, proportion of cygnets, brood size and food choice of a large proportion of the western population of Bewick ’s Swans have been established in the whole Netherlands on a bi-weekly basis throughout the winters 1982-83 and 1983-84. From November to March at least 6,000 were present, including a peak of nearly 9,000 in January (55 % of the flyway-population). Patterns in the proportion of cygnets can be explained by families arriving later in autumn, and cygnets departing later in spring. On arriving in autumn the swans prefer feeding on pondweed tubers in aquatic habitats. Having depleted these they switch to arable land (root crops, winter wheat) and grassland. After January only grassland is utilized. The switch from food types with a high carbohydrate content (pondweed tubers, sugar-beet, potato) to protein-rich food (winter wheat, grasses) is discussed. The sequence in food choice through the winter season is compared with the history of food choice of Bewick ’s Swans in the Netherlands in which major changes have occurred in the last few decades.