KLAASSEN O (2012) Increase of wintering Great White Egrets Casmerodius albus in The Netherlands as shown by diurnal counts and roost counts. LIMOSA 85 (2): 82-90.
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.
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