KLEEFSTRA R, DE JONG, J & SCHAUB T (2023) Breeding of Short-eared Owls Asio flammeus in fields full of Common Voles Microtus arvalis in the province of Friesland in 2019. LIMOSA 96 (2): 60-71.
When in 2014 an outbreak of Common Voles led to an
invasion of about 50 breeding pairs of Short-eared Owls
in the Frisian agricultural grasslands, it was believed
to be a once-in-a-lifetime natural history event. In the
winter of 2018/19, however, signs of a vole outbreak
became apparent and already in March the first Shorteared Owls started breeding.
A total of 85 probable
breeding cases was reported. The breeding period
covered over half a year, from the end of February
(first eggs) to almost mid-September (fledging of the
last young). In 10 cases there was evidence for second
clutches (pairs starting a new clutch shortly after a
failed breeding attempt in the vicinity of the first nest),
bringing the number of territories to at least 75 (Tab.
1). This number is significantly higher than in 2014 and
accounts for the highest number of breeding Shorteared Owls in the province Friesland since 1990 (Fig. 1).
The Short-eared Owls spread widely over the western
and northern half of the province (Fig. 2). As in 2014,
slightly more than half of the nests were located on
peat soil (56.0%; Tab. 1) and by far the vast majority
(90.5%) bred in intensive agricultural grassland with
The average number of eggs in 34 complete clutches
was 7.5 eggs per nest (Tab. 2). Compared to 2014,
clutches were larger by on average two eggs per nest.
Based on nests from which this could be calculated,
Short-eared Owls started laying between 21 March and
2 July (except one clutch that must have been started
as early as late February). Calculated over all clutches
egg laying started 23 days earlier than in 2014. The first
young of 13 nests hatched between 14 April and 28 July,
with 5 June as the average date. Of 231 eggs in 38 nests,
at least 115 (49.8%) hatched in 17 nests. This is lower than
in 2014 when 56.3% of 167 eggs hatched. The number
of young that fledged was 98-150, of which 94 were
ringed. This is more than in 2014 when 82-112 young
fledged, 64 of which were ringed. Although the number
of territories and the number of fledglings in 2019 was
higher than in 2014, the number of successful nests
was the same (Tab. 3). The chick condition was similar
to that in 2014 (Fig. 3). The reason for nest failures was
known for 31 (partly first) breeding attempts (Tab. 3). At
least 14 nests were predated, of which five within a day
after mowing of the grass. In three cases the predator
was a small marten, in one case a Carrion Crow and in
one case the breeding female was predated by Red Fox
or Beech Marten. Among nine abandoned clutches,
three were no longer incubated immediately after
mowing. Of eight nests the cause of failure could not
be determined with certainty, but mowing is the most
likely cause. In at least one case, a brooding female
was killed during mowing. The picture on causes for
breeding failures does not differ much from 2014 (Tab.
During nest visits, a total of 101 pellets were collected,
in which the majority (95.8%) were vole and the other
prey also consisted of small mammals (Tab. 4). In one
region, 24 fresh, intact voles with an average weight
of 26.5 grams (range 15-35 grams), were found during
three nest visits, divided over two nests.
Short-eared Owls nested at locations with relatively
high densities of Common Voles (Tab. 5). This is
expressed not only in the average number of holes per
square meter (plot), but also in the percentage of plots
in which fresh tracks of vole were present. This was
the case in about 85% of the 360 plots. In autumn the
density of holes had doubled.
We tagged an adult male Short-eared Owl on 18 July,
2019. It stayed at the nesting site until the end of August
(its chicks then 8-9 weeks old). He then moved 2 km to
the west, only to move two weeks later over 4 km east
of the nesting site, where he stayed for at least a month.
During the nesting period (18 July to 30 August) he had
a small home range of 44 ha with a core area of 3 ha (Fig.
4). 97.5% of positions were within 500 m of the nest;
the maximum distance was 2.2 km (median 52 m). The
transmitter’s battery ran out in mid-October. When the
transmitter reconnected in January 2020, it turned out
that the Short-eared Owl had moved to Libya since 11
December, 2019. At the end of January, it moved to the
border area between Tunisia and Algeria. In early April
2020, it took off in a north-easterly direction and flew
in a straight line via Sicily and southern Italy, through
the Balkans to Russia (Fig. 5). Between the end of May
and the end of June it was stationary in a small area in
Saratov Oblast (about 600 km south-east of Moscow).
It is conceivable that he made a (failed) breeding
attempt here, but without field observations this is
uncertain. The last GPS position was from Bulgaria on
25 November, 2020. After that, the transmitter made
regular GSM connections, but without sending GPS
coordinates. From the last two connections in August
2021, it could be derived that the owl was again in
Russia (Russian network provider). It is not certain
whether the Short-eared Owl was still alive at the time.
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