Wintering Great White Egrets Ardea alba in a riverine area in the Netherlands: numbers, abundance and timing

Abstract

ABSTRACT Capsule The riverine area in the Netherlands is an important wintering area for Great White Egrets Ardea alba. Numbers decline in relation to very cold weather, and increase in relation to peaks in population cycles of Common Vole Microtus arvalis. Aim To analyse the variation in abundance and timing of wintering Great White Egrets between years and roosts. Methods The study area was a 21,000 ha section of the riverine area between the rivers Meuse and Waal. The paper presents the results of eight years of weekly counts of three roosts, one year of 10 roosts and seven years of nearly daily counts of foraging birds in an agricultural landscape. All surveys were carried out outside the breeding season. Results Birds used the area from the end of June until the beginning of April. Abundance strongly varied with small rodent prey availability, with maxima between 15 and 50 birds in years with average prey abundance, and a maximum of 125 individuals in 2019/2020 when prey availability was high. Since most birds foraged on grassland, this suggests a density of two birds per 100 ha in normal years and eight birds per 100 ha in peak years. Each roost covers a foraging area of 2000 ha in total, or 1000 ha grassland. Mean densities were similar between years but there was seasonal variation, with most roost counts peaking during autumn. Birds would temporarily leave the area during spells of snow or frost. In spring, departure towards the breeding grounds was partly dependent on the mean temperature in March. Conclusion Counts of Great White Egrets at roosting and foraging sites in the Netherlands vary seasonally with temperature and prey availability.