Between year variation and causes of mortality in the non-breeding population of the Mute Swan Cygnus olor in the Netherlands, with special reference to hunting

Abstract

The non-breeding population, mainly young and sub-adult birds, of the Mute Swan in the Netherlands concentrates each year during the summer in lakes IJsselmeer and Grevelingen, where they moult in large flocks. The numbers of these moulting birds vary considerably from year to year. For instance on Lake IJsselmeer between 1983 and 1987 a decline from 4,750 to 2,870 birds was recorded. With data on reproductive success, survival of young birds in their first year (banding data) and mortality differences between severe and mild winters (banding data) this paper attempts to analyse the main factors causing these differences in the non-breeding population. The human impact through shooting swans (20-25% of the Dutch population annually) and disturbing breeding attempts (30-40% annually) for protection of agricultural interests is also covered.