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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