VAN BOEKEL W & JANSEN M (2022) Numbers and diet of Hen Harriers Circus cyaneus at a winter roost in nature reserve De Onlanden. LIMOSA 95 (3): 145-153.

In 2012, De Onlanden in northern Netherlands, was converted from extensive agricultural grassland to marshland by raising the water level. Hen Harriers were known to winter there communally since at least 2008. After raising the water level the harriers kept roosting in the area, at (almost) the exact spot. In 2014-20 (with the exception of the winter of 2016/17) numbers of birds arriving at the roost were counted regularly (Fig. 1). Distinction was made between (sub)adult males and ringtails (including all females and juvenile males). Each winter, 400-900 pellets were collected regularly and analysed for prey remains (Tab. 1).
The first harriers arrived in October and maxima were reached in January. By the end of March the roost was deserted. The mean percentage of males at the roost was 28%, and showed little variation over the season and years. The maximum number of birds at the roost was relatively high during the last two winters, possibly due to high densities of Microtus arvalis in the north of the Netherlands. In most winters the diet of the Hen Harriers consisted primarily of voles, especially M. arvalis and to a lesser extent M. agrestis. Birds usually formed 5-20 % of the prey-items, with the exception of the season 2016/17 (Fig. 2). In this winter the Hen Harriers relied for a far larger part on birds in their diet, probably due to a low availability of M. arvalis in the region. In weight, birds formed almost 60% of the Hen Harrier's diet during that winter (Fig. 3).

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