KLEEFSTRA R, BARKEMA L, VENEMA DJ & SPIJKSTRA-SCHOLTEN W (2015) A plague of voles, an invasion of breeding Short-eared Owls Asio flammeus in the province of Friesland in 2014. LIMOSA 88 (2): 74-82.

With a breeding population of 30 pairs of Short-eared Owls in the Netherlands in recent years, we were surprised to find as many as 80 pairs in the spring of 2014. More than half of these pairs bred in the interior of the province of Friesland (Fig. 1), where in a vast area Common Voles Microtus arvalis reached plague densities (Fig. 2). Most Short-eared Owls (N=31) bred in intensively used farmland and started laying between 16 April and 25 June (Tab. 2). Some nests were found too late and failed due to mowing, but at least 13 pairs started a second brood (Tab. 1). Finally 21 pairs produced 82-112 fledged young, of which 64 were ringed. Pellets and fresh prey remains in nests indicated that the diet consisted almost entirely of Common Voles. On one evening a male was observed to catch 11 voles during an hour around sunset in an area of less than 20 hectares, and to divide 10 of these between its five young in a nearly fixed order. Despite the high vole densities the condition of the young (based on weight/wing length ratio) was similar to that of young owls in a French study (Fig. 3).
The owls may have originated from Northern Scandinavia, where the species experienced a very successful breeding season in 2011. Being both migratory and nomadic, these birds probably were responsible for the influx in the UK in autumn 2011 and the higher numbers observed in The Netherlands since 2011. Part of those birds still alive in 2014 may have discovered the Frisian meadows, perforated with vole burrows. The causes of the vole outbreak, which was the largest in decades and lasted for more than a year, remain unclear. They might be a combination of the extremely warm and dry winter, spring and summer, the deep drainage and the low stocking density in the Frisian meadows.

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