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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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limosa 89.2 2016
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