Bewick's Swan Cygnus columbianus bewickii utilising the changing resource of Potamogeton pectinatus during autumn in the Netherlands

Abstract

Returning from the breeding grounds, Bewick's Swans show a clear preference for Sago Pondweed tubers. For two lakes in the Netherlands, which may hold up to almost half of the entire flyway population, data on timing of arrival, exploitation patterns, and bird numbers in relation to available food stocks are presented. The swans depleted the tuber stocks down to a certain threshold value, below which no grazing occurred. Estimates of daily energy intake were in accordance with field metabolic rate. Quantifying food stocks by means of aerial photography and detailed sampling reveals an adequate estimate of carrying capacity. The importance of the availability of high quality macrophyte food during autumn migration is stressed, and discussed against the background of the apparent scarcity of suchfood sources along the flyway of this population. We suggest the necessary existence of stopover sites during autumn somewhere in the NE Baltic/White Sea region.