GERRITSEN G.J. (2016) Vole-eating Great White Egrets Casmerodius albus in the Mastenbroek polder in 2007-2015. LIMOSA 89 (2): 75-78.
The Great White Egret winters in increasing numbers in
the Netherlands. In 2014/15 an unexpected large number
of wintering egrets were counted (8900 birds), which was
related to an outbreak of Common Voles Microtus arvalis in
the northern part of the country. In this paper I present the
results of observations on vole-hunting Great White Egrets
in the Mastenbroek polder (65 km2) near Zwolle during
eight winters (2006/07-2014/15). In this period Common
Voles were abundant during two winters; 2007/08 (high
numbers) and 2014/15 (outbreak). The number of wintering
egrets was directly related to vole abundance. The egrets
strongly preferred grasslands with a short sward. In August October
they selected recently mowed fields whereas
from October-November onwards they preferred grazed
meadows (cattle, sheep and horses). In the two peak winters
the egrets fed mostly on voles, although not exclusively. In 2007/08 the proportion of fishing egrets was slightly higher
than in 2014/15 (32 versus 11%). In 2014/15 time budgets were
determined. Dominant behaviour of hunting egrets consists
of standing still (47% of the time), walking (38%) and ‘strike
posture’ (13%). The time a Great White Egret needed to
capture a vole was directly related to vole abundance: 20.3
minutes in winters with low vole numbers, 11.6 minutes in
2007/08, and 8.5 minutes in the vole outbreak winter 2014/15.
This was a direct effect of a higher strike rate as the strike
success was similar between winters (57%). Based on a daily
intake of almost 22 voles, I estimate that the Great White
Egrets consumed a total of slightly over a million voles in the
Mastenbroek polder in 2014/15
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