ArtikelDe toename van overwinterende Grote Zilverreigers in Nederland aan de hand van dagtellingen en slaapplaatstellingen

Omslag De toename van overwinterende Grote Zilverreigers in Nederland aan de hand van dagtellingen en slaapplaatstellingen

This paper describes the development of numbers of Great White Egret wintering in The Netherlands since 2000/01, based on multiple data sources: non-systematic observations, monthly waterbird counts and dedicated counts at night roosts. The latter were initiated in 2003/04 but national coordination and coverage was only achieved some years later (Tab. 1). In recent years, Great White Egrets are found wintering throughout the country (mainly in farmland and marshlands), but the regions that were occupied first still hold the highest densities (Fig. 3). Within the wintering period movements must occur, since different provinces show of four different seasonal patterns, with a peak in autumn (3 provinces), in mid-winter (3) or at the end of the winter (5).

The province holding the only Dutch breeding colony, Flevoland, shows a very different pattern with the lowest numbers in (early) winter (Fig. 4). Foraging behaviour changes in the course of the non-breeding season, with aquatic foraging predominant in autumn (when egrets sometimes follow fishing flocks of Great Cormorant Phalacrocorax carbo) followed by a switch to land-based foraging in winter, with Common Voles Microtus arvalis as an important prey.

The strongly increasing trends in numbers based on nonsystematic observations and waterbird counts are very similar (Fig. 1). Roost counts yielded the highest numbers, exceeding those from the waterbird counts by 32% on average during the four most recent winters. The number of known roost sites has also increased, particularly that of small roosts with less than 10 birds (Fig. 5). Between 2003/04 and 2010/11, in total 221 roost were found, among which 30 with more than 50 birds, including 9 with more than 100 (maximum at a single roost 917 during a frost in February 2012). Of the three data sources, the waterbird counts provide the most reliable trend indices and seasonal patterns, the roost counts the most comprehensive population estimates, and the nonsystematic observations add significantly to the distribution pattern. An estimate for the national wintering population in 2010/11 arrived at 2300-2800 birds. This alone indicates that the growing Dutch breeding population, currently comprising about 150 pairs, cannot be the single source of the increase.

This is corroborated by a growing number of colour ring resightings, mainly from France but also from Poland. From data on breeding numbers and ringing results elsewhere in Europe, a pattern emerges of a decreasing traditional breeding population in Austria/Hungary with a southern winter range, and newer more northerly breeding populations in France, The Netherlands, Poland, Ukraine and Belarus which show a strong increase and of which the birds winter at roughly the same latitudes, but partly make extensive westward movements.