Content uploaded by Geir A. Sonerud
Author content
All content in this area was uploaded by Geir A. Sonerud on Jun 29, 2021
Content may be subject to copyright.
VOLUME 29 2021
442
Home range, perching height and reaction
to approaching humans by radio-tagged
Ural Owls
Área vital, altura dos poisos e reação à aproximação de humanos
por parte de corujas dos Urales marcadas com rádio-emissores
Four breeding Ural Owls (Strix uralensis) (one pair, one female and one male) were radio-
tagged at the nest and tracked on foot with portable equipment in Hedmark county in SE Nor-
way. The owls´ positions were determined by cross-triangulating or by direct observations. A
total of 105 plotted locations were obtained. The mated male and female were located 58 and
22 times on 30 and 17 separate days, respectively, in 1989, while the other female was located
18 times on 15 separate days in 1989, and the other male 7 times on 4 separate days in 1990.
From October 1989 on, presumably after the young became independent, the rst male moved
out of his summer range and eastwards into Sweden. Home range areas were treated as summer
areas until this date, and winter areas thereafter. Calculated as 100% minimum convex polygon,
the summer (May - September) home range for this male was 11 km2, while his winter (October
- December) home range was 63 km2. The corresponding home ranges for his mate were 7 km2
ABSTRACT
* Corresponding author: roar.solheim@uia.no
Roar Solheim1,*, Geir A. Sonerud2,
Hallvard Strøm3
1 Natural History Muse um, Agder Universit y, P.O. Box 422,
N-4604 Kristiansand, Norway
2 Faculty of Environmental Sciences and Natural Resource
Manageme nt, Norwegian University of Life Science s, P. O.Box
5003, N-1432 Ås, Norway
3 Norwegian Polar Ins titute, Framsenteret, P.O. Box 6606
Langnes , N-9296 Tromsø
443
Área vital, altura dos poisos e reação à aproximação de humanos por corujas dos Urales
Quatro corujas dos Urales (Strix uralensis) reprodutoras (um casal, uma fêmea e um macho)
foram marcadas com rádio-emissores no ninho e seguidas a pé com equipamento portátil no
condado de Hedmark, no sudeste da Noruega. As posições das corujas foram determinadas por
triangulação ou por observações diretas. Foram obtidas 105 localizações. O macho e a fêmea
acasalados foram localizados 58 e 22 vezes em 30 e 17 dias diferentes, respetivamente, em 1989,
enquanto a outra fêmea foi localizada 18 vezes em 15 dias em 1989, e o outro macho 7 vezes
em 4 dias em 1990. A partir de outubro de 1989, presumivelmente depois de os juvenis se terem
tornado independentes, o primeiro macho abandonou a sua área de verão e deslocou-se para
leste, para a Suécia. As áreas vitais foram tratadas como áreas de verão até essa data e, posterior-
mente, como áreas de inverno. Calculada como polígono convexo mínimo de 100%, a área vital
de verão (maio - setembro) desse macho foi de 11 km2, enquanto a área vital de inverno (outubro
- dezembro) foi de 63 km2. As áreas vitais correspondentes da sua companheira foram de 7 km2 e
20 km2, enquanto para a outra fêmea foram de 27 km2 e 32 km2. A área vital total para o período
de seguimento foi de 112 km2 para o macho acasalado, 42 km2 para a sua companheira e 40 km2
para a outra fêmea. Ocorreu uma sobreposição negligível entre as áreas vitais das duas fêmeas a
nidicar a 8 km de distância. A análise kernel das localizações do macho durante o verão reve-
lou que este passou metade da sua atividade numa área de 5 km2 em torno do ninho. As corujas
usaram poisos entre 2,3 e 8,0 m acima do solo, com uma média de 5,0 m, geralmente na metade
inferior da árvore. As corujas levantavam frequentemente voo antes de serem avistadas. Nas
ocasiões em que as corujas foram avistadas e fugiram, a distância de início do voo variou entre
8 e 35 m, com uma média de 20 m. Não foram encontradas regurgitações debaixo dos poisos, e
uma coruja foi encontrada a ingerir uma presa apenas uma vez.
RESUMO
Palavras-chave: altura do poiso, área vital, reação à aproximação de humanos, Strix uralensis, telemetria
Keywords: Home range, perching height, reaction to human approach, Strix uralensis, telemetry
and 20 km2, while for the other female they were 27 km2 and 32 km2. Overall home range for the
whole tracking period was 112 km2 for the mated male and 42 km2 for his mate, and 40 km2 for
the other female. There was negligible overlap between the home ranges of the two females nest-
ing 8 km apart. A kernel analysis of the male´s summer range data showed that he spent half his
activity within an area of 5 km2 around his nest. The owls perched 2.3 - 8 m above ground, with
an average of 5.0 m, usually in the lower half of the perch tree. The owls often took off before
they were spotted. On the occasions when the owls were seen and ushed, the ight initiation
distance ranged 8-35 m, with an average of 20 m. No pellets were found below the perches, and
only once was an owl located when ingesting a prey.
444
Home range, perching height and reaction to approaching humans by Ural Owls
The Ural Owl (Strix uralensis) is a cavi-
ty-nesting prey generalist that inhabits the
northern taiga from Japan across Russia to
Fennoscandia, and its westernmost distribu-
tion reaches across Sweden into Hedmark
county in SE Norway (Mikkola 1983, Cramp
1985). Before 1985 very few nests had been
found in Norway (Solheim 1985). In 1979
a nest box study was initiated, and from
1985 a total of 86 nest boxes were available
in potential Ural Owl habitat in Hedmark
county (Solheim et al. 2009). The sparse data
recorded in Norway before 1985 indicated
that Ural Owl clutch sizes were smaller than
those reported from Sweden and Finland (Sol-
heim & Bjørnstad 1987). We thus suspected
that food availability could be a limiting fac-
tor resulting in smaller clutch sizes in Nor-
way. In Finland Ural Owls show a functional
response to microtine rodents, and otherwise
prey on shrews, hares, birds and frogs (Kor-
pimäki & Sulkava 1987). Because Ural Owls
in Fennoscandia do not migrate during win-
ter and tend to reside within their established
territories, they may be more exposed to poor
hunting conditions during harsh winters and
low microtine rodent populations. According
to Lundberg (1980, 1981), the availability of
microtine rodents during winter and early
spring is the main regulatory factor for breed-
ing success of Ural Owls in Sweden. Solheim
& Bjørnstad (1987) thus proposed to use
radio telemetry to follow adult Ural Owls
in SE Norway during autumn and winter to
study their hunting habits and diet. To this
end, three nesting Ural Owls were tted with
radio transmitters in 1989 and one in 1990,
and located until the transmitters expired.
Here, we present results on home range size
and perching behaviour from this study.
Introduction Methods
In 1989 two Ural Owl nests situated 8.3
km apart were located at c. 60°46´N, 12°16´E
in Hedmark county in SE Norway. One nest
was located in a cavity in a partly dead aspen
(Populus tremulus) (loc. 1), which was rst
found with nesting Ural owls in 1984. The
other nest was in one of the project nest
boxes (loc. 2), which had been available but
unoccupied since autumn 1979. In each nest,
two young edged c 1 June. We used mist
nets to capture the adults on 19 May (male
1; body mass 720 g), 1 June (female 2; body
mass 815 g) and 8 June (female 1; body mass
840 g). Male 1 and female 1 were paired,
while we did not succeed in capturing the
mate of female 2. In 1990, a new male (here
denoted as male 3) had occupied loc. 1 and
nested with female 1. He was captured on 19
May (body mass 733 g).
The owls were tted with a transmitter
(model Biotrack SR-1) mounted as a back-
pack and attached by tubular Teon tape
(Bally Ribbon Mills, PA, USA) as harness. The
total backpack including harness weighed c.
20 g, thus amounting to less than 2.7 % of
the body mass of the lightest owl. The trans-
mitters had a mercury switch which changed
the signal to a more rapid pulse when the
owls were ying or leaning forward, thus
making it easier to cross-triangulate the owls
and interpret their behaviour.
The owls were cross-triangulated during
day-time to locate roost sites, and tracked
during the evening and night hours to locate
their hunting grounds and to watch their
hunting behaviour. This was done on foot
using a portable receiver (model Televilt
RX-81) and a hand-held 4-element yagi-an-
tenna. When tracking the owls during daytime
and signals indicated that they were roosting,
we usually tried to approach the owl at an
445
Área vital, altura dos poisos e reação à aproximação de humanos por corujas dos Urales
angle to circle it and avoid ushing it before
it could be spotted. Whenever the owls were
seen perched, we noted the perching height.
Locations of the owls were marked on cop-
ies of aerial photos. By using modern digital
maps, web-based aerial photos, and Google
earth, the locations were recently transferred
to Google earth maps. This gave GPS data
for each marked location and allowed mod-
ern analyses of the owls´ home ranges. Male
1 was located from 20 May until 26 Decem-
ber, while female 1 and female 2 were tracked
from 16 June until 26 December, and from 15
June until 9 December, respectively, in 1989.
Male 3 was located from 19 May until 15
June in 1990. Young Ural Owls are usually
dependent on their parents´ feeding efforts
until late August, after which they start dis-
persing (Valkama et al. 2014). We treated
locations during May-September as repre-
senting summer home range of the owls, and
locations obtained during October-Decem-
ber as representing winter home range. All
locations of the females were made after the
young had edged. The transmitters on male
1 and female 1 had expired on 7 January
1990, and that on female 2 on 20 December
1989. The transmitter on male 3 had expired
prematurely on 30 June 1990.
We obtained a total of 98 locations in 1989
and 7 locations in 1990. Male 1 and female
1 (the mated pair) were located 58 and 22
times on 30 and 17 separate days, respec-
tively, while female 2 was located 18 times
on 15 separate days. Male 3 was located only
7 times on 4 separate days, so home range
size was not calculated.
Of the locations of male 1, approximately
half were termed diurnal (made when the sun
was above the theoretical horizon) and half
were termed nocturnal (made when the sun
was below the theoretical horizon. Of the
locations of the two females, approximately
two third were diurnal.
The time interval between successive loca-
tions of the same owl varied from 2 minutes
to 34 days 22 hours, with a median of 12
hours 29 minutes. Because 98% of the inter-
vals were longer than 5 minutes, and 90%
were longer than 21 minutes, we regard the
locations as fairly independent.
Home ranges were calculated by use of
two methods in the package “adehabitatHR”
in the statistical software R (R Development
Core Team 2017); minimum convex poly-
gon (MCP) and kernel (Wolton 1989). The
former was used to enable comparison with
a previous study. Means are presented with
standard error (SE).
Results
When home range was calculated as 100%
MCP it covered 41.9 km2 for female 1 and
39.7 km2 for female 2, with hardly any over-
lap between these two neighbours, and 112.0
km2 for male 1 overall (Fig. 1). When it was
calculated as separate 100% MCP ranges for
the offspring dependence period (May - Sep-
tember) and the ensuing non-breeding period
(October - December). it covered 11.2 km2
(49 locations) and 62.8 km2 (9 locations),
respectively, for male 1, with no overlap (Fig.
1). The male had moved 9 km between the
last location in the offspring-dependence
period (25 September) and the rst location
in the non-breeding period (2 October). For
female 1 the corresponding ranges were 7.3
km2 (16 locations) and 19.5 km2 (6 loca-
tions), while for female 2 they were 26.7 km2
(11 locations) and 31.7 km2 (7 locations),
respectively.
The number of locations of the females
was too small to calculate home range as ker-
nel. The same was the case for the male in the
non-breeding period. For the male during the
offspring dependence period (May - Septem-
446
Home range, perching height and reaction to approaching humans by Ural Owls
ber) 95%, 75%, 50%, 25% and 10% kernel
range covered 21.8, 11.2, 5.4, 1.9 and 0.6
km2, respectively (Fig. 2). His nest was located
within the 25% (and higher) kernel range, but
outside the 10% kernel range (Fig. 2).
When we approached perched owls during
daytime, they either took off before we could
see them or at the moment we spotted them,
or they stretched out in camouage posture
trying to avoid detection. On 24 occasions
the owls were observed before they took
off, and their perching height could be mea-
sured. The owls were usually perching in the
lower half of the tree, typically on the lowest
thick branches of a pine, and were never seen
perching in the top of a tree. Perching height
was on average 5.0 ± 0.3 m and ranged 2.3
- 8 m, with no signicant variation between
the four individuals (F3,20 = 0.29, p = 0.83). In
seven of these cases we measured the height
of the perch tree, and it ranged 6 - 15 m, with
an average of 10.8 ± 1.4 m. The perching
height relative to the tree height ranged 0.30
– 0.83, with an average of 0.48 ± 0.06.
On the 14 occasions when the owls were
seen and ushed, the ight initiation distance
ranged 8-35 m, and was on average 19.7 ±
2.4 m, with no signicant variation between
the four individuals (F3,10 = 1.72, p = 0.23).
This ight initiation distance was not signi-
cantly related to perching height (F1,12 = 0.37,
p = 0.55).
We did not attempt to measure the owls´
habitat preferences, i.e. habitat use against
available habitat types. During the seven
months we were able to follow the Ural
Owls, we did however get a good impression
of where the owls moved while hunting. They
were often found in swampy pine or mixed
coniferous forest, and never seen perching in
open areas like bogs or clear-cuts. When the
male of the mated pair was ying towards
the nest, he crossed some clear-cuts usually
in tree-top height. When the owls were ying
2.5 0 2.5 5 7.5 10 km
Figure 1 - Home ranges of the three Ural owls radio-tracked in 1989, expressed as 100% MCP, with each plotted location
shown. Green colour denotes the range of the male during May - September, and blue colour his range during October -
December. The overall range of his mate (female 1) is denoted by purple colour, with locations from June - September in red
colour and locations from October - December in purple colour. The overall range of the other female (female 2) is denoted by
orange colour, with locations from June - September in orange colour and locations from October - December in grey colour.
Figura 1 - Áreas vitais das três corujas dos Urales seguidas por rádio-telemetria em 1989, expressas como 100% MCP, com
cada localização representada. A cor verde indica a área ocupada pelo macho entre maio e setembro, e a cor azul a área ocupada
entre outubro e dezembro. A área ocupada pela sua companheira (fêmea 1) é indicada pela cor roxa, com as localizações de
junho a setembro a vermelho, e as localizações de outubro a dezembro a roxo. A área ocupada pela outra fêmea (fêmea 2) é
indicada pela cor laranja, com as localizações de junho a setembro a laranja e as localizações de outubro a dezembro a cinza.
447
Área vital, altura dos poisos e reação à aproximação de humanos por corujas dos Urales
in forest, they often ew close to the ground
(0.5-1.5 m).
We did not nd any pellets at the sites where
owls were located perching during daytime.
The movements of the owls indicated that they
did not establish specic roost sites after the
chicks had left the nest, and ended up resting
wherever their nightly hunting had brought
them at the dawn of day. On the very last
tracking day we located female 1 with prey.
When we triangulated her after 11.30 pm in
the dark in snowy weather on 26 December,
the transmitter signals indicated that she was
eating (alternating quick and slow pulse of sig-
nals from a stationary position). The owl was
ushed from a carcass of a female Capercaillie
(Tetrao urogallus) on the ground. The bulging
eyes of the carcass indicated that the prey was
recently killed, and thus most likely killed by
the Ural Owl herself.
Discussion
Home range
During May – September 1989, when pro-
viding for offspring, the Ural Owl male had a
home range covering 11 km2 when calculated
as 100% MCP. In comparison, a Ural Owl
male radio-tracked for 9 weeks during May -
July in 1991 in Värmland county in Sweden,
in the same boreal forest region as our study
area and less than 100 km towards south-
east, had a 100% MCP home range covering
21 km2 (Bolin et al. 1992). These areas were
5-10 times larger than those found for 100%
MCP home ranges of providing males of
the Boreal Owl (Aegolius funereus) and the
Northern Hawk Owl (Surnia ulula) in boreal
forests c. 100 km northwest of our study area
in 1983-85 (Sonerud et al. 1986, Bækken et
750 0 750 1500 2250 3000 m
Figure 2 - Home range of the Ural owl male during May - September, expressed as kernel utilization distributions, with each
plotted location shown in green colour. Brown colour denotes 10%, yellow 25%, green 50%, light blue 75%, and dark blue
95% utilization. The red star denotes the position of the nest.
Figura 2 - Área do macho de coruja dos Urales durante os meses de maio a setembro, expressa em distribuições de utilização
de kernel, com cada localização estimada na cor verde. A cor castanha indica 10%, amarelo 25%, verde 50%, azul claro 75%
e azul escuro 95% de utilização. A estrela vermelha indica a posição do ninho.
448
Home range, perching height and reaction to approaching humans by Ural Owls
al. 1987, Jacobsen & Sonerud 1987). The two
latter species are smaller than the Ural Owl,
so would be expected to need smaller ranges,
given the same access to prey. Moreover, the
Boreal Owl and the Northern Hawk Owl are
more specialized on small mammals as prey
than is the Ural Owl (Mikkola 1983, Cramp
1985), and therefore depend on higher small
mammal abundance to nest. Thus, the home
range estimates obtained for Boreal Owls
and Hawk Owls were probably on average
representing higher small mammal abun-
dances than were those obtained for the Ural
Owl. In the low vole year 1993, Eurasian
Pygmy-owl (Glaucidium passerinum) males
in a mixed boreal forest-farmland area c. 100
km northwest of our study area had 100%
MCP home ranges averaging 3 km2 (Strøm &
Sonerud 2001).
The kernel analysis of the locations
obtained for the Ural Owl male suggests that
during the provisioning period he was as
likely to be located within an area of 5 km2
around the nest as further away. Thus, a sub-
stantial part of his movement seemed to be
concentrated on a rather small area.
The overall home ranges of the two Ural
Owl females (100% MCP) had strikingly
similar size (40 km2 and 42 km2). It is also
striking that these neighbours, nesting 8 km
apart, had 100% MCP home ranges just
touching each other, with minimal overlap.
Ural Owl males are strongly territorial (Mik-
kola 1983, Cramp 1985), and our data sug-
gest that this applies to the females as well.
While one of the females had a smaller 100%
MCP home range in the offspring-depen-
dence period (7 km2) than in the non-breed-
ing season (19 km2), this was not the case for
the other female (27 km2 and 32 km2, respec-
tively). The former female had a similar home
range size in the offspring-dependence period
as her mate (7 km2 and 11 km2, respectively).
Young Ural Owls depend on their par-
ents for food until late August (Valkama et
al. 2014). This may explain why the male
Ural Owl did not start expanding his home
range until October, and thereafter moved
20 km towards the east into Sweden. His
mate nested in the same cavity in the aspen
tree with a new male (male 3) in 1990, but
thereafter she relocated 1 km eastwards to
a nest box, which she used for breeding in
later years. Thus, the male may have been
evicted from his territory by the new male in
fall 1989.
Perching height
The Ural Owls did not tolerate humans
at close distance. We were thus unable to
closely follow the Ural Owls and observe
their hunting behaviour, as is possible for
Boreal Owls (see Bye et al. 1992). On aver-
age, the Ural Owls perched 5 m above
ground when located. In comparison, radio-
tagged Boreal Owls on average perched 3 m
above ground when hunting at night, and 4
m above ground when roosting during day-
time (Bye et al. 1992). We did not discrimi-
nate perches used for hunting from perches
used for roosting by the Ural Owls, but
apparently they perched higher than Boreal
Owls when hunting. Although the Ural Owl
is less constrained to nocturnal hunting than
is the Boreal Owl, both use auditorial cues to
locate prey (Cramp 1985). Because Ural Owl
males are twice as large as Boreal Owl males
by linear body measures (Cramp 1985), the
distance between their ears is larger, so they
would be able to determine the exact position
of a ground-dwelling prey by acoustic cues at
a larger distance (cf. Norberg 1970, 1978).
Prey
Unfortunately, we were unable to doc-
ument prey taken by the radio-tagged Ural
Owls to reveal their diet during fall and win-
ter, with one exception (see below). The owls
obviously did not use permanent roost sites
outside the breeding season, and pellets were
thus very hard to nd. The way Ural Owls
seemed to avoid close contact by humans
449
Área vital, altura dos poisos e reação à aproximação de humanos por corujas dos Urales
made it even more difcult to locate roosts
sites and pellets. This shy behaviour was
quite similar to that found in radio-tagged
individuals of the closely related Tawny Owl
(Strix aluco) and Great Grey Owl (Strix neb-
ulosa) (pers. obs.).
The only prey item we were able to record
was a female Capercaillie being ingested by
female 1 close to midnight in late Decem-
ber. This prey weighs c. 2 kg, which is 2-3
times as much as a female Ural owl (c. 800
g). By feeding mammalian and avian prey to
owls in temporal captivity, and video record-
ing the owls´ prey handling, Slagsvold &
Sonerud (2007) and Slagsvold et al. (2010)
found that ingestion rate decreased with prey
size, and was higher for mammalian than for
avian prey. From gs. 2 and 3 in Slagsvold &
Sonerud (2007), we estimated that a micro-
tine rodent weighing 20 g, thus c. 2.5% of a
female Ural Owl (800 g), would be ingested
at a rate of 30 g/min, thus in 40 s, whereas
an avian prey weighing more than twice as
much as the owl would be ingested at a rate
of less than 1g/min. Although only c. 70%
of the body mass of the latter prey would be
consumed (g. 1 in Slagsvold et al. 2010), it
would still take the owl c. 24 hours or more
to consume the prey. Thus, the probability of
locating a Ural Owl while it is consuming a
microtine rodent is negligible compared to
the probability of locating it when it is con-
suming a female Capercaillie. This would
explain why the only prey we recorded was a
female Capercaillie, and why we never found
the owls while they were consuming smaller
prey.
Acknowledgements
We thank Kjell Isaksen and the late Torger
Hagen for participating in the tracking of the
Ural Owls, Tomas Aarvak for help with the
GIS analyses, and Ronny Steen for calculat-
ing the home ranges and making the gures.
The manuscript was improved by critical
comments from Pertti Saurola. Trapping of
the owls and mounting of radio transmitters
complied with Norwegian law.
References
Bækken, B. T., Nybo, J. O. & Sonerud, G.
A. 1987. Home range size of hawk owls:
dependence on calculation method, num-
ber of tracking days, and number of plot-
ted perchings. In: Nero, R. W., Clark, R.
J., Knapton, R. J. & Hamre, R. H. (eds.).
Biology and conservation of the northern
forest owls: symposium proceedings, pp.
145-148. US Department of Agriculture,
Forest Service, General Technical Report
RM-142, Fort Collins, Colorado.
Bolin, J., Fuglsang, M. & Hellquist, J. 1992.
Biotoputnyttjande hos häckande slaguggla
Strix uralensis. Värmlandsornitologen 20:
5-10. (In Swedish).
Bye, F. N., Jacobsen, B. V. & Sonerud, G. A.
1992. Auditory prey location in a pause-
travel predator: search height, search time
and attack range of Tengmalm's owls
Aegolius funereus. Behavioral Ecology 3:
266-276.
Cramp, S. (ed) 1985. Handbook of the birds
of Europe, the Middle East and North
Africa. Vol. IV. Oxford Univ. Press, Oxford.
Jacobsen, B. V. & Sonerud, G. A. 1987. Home
range size of Tengmalm's owl: a compar-
ison between nocturnal hunting and diur-
nal roosting. In: Nero, R. W., Clark, R.
J., Knapton, R. J. & Hamre, R. H. (eds.).
Biology and conservation of the northern
forest owls: symposium proceedings, pp.
189-192. US Department of Agriculture,
Forest Service, General Technical Report
RM-142, Fort Collins, Colorado.
VOLUME 29 2021
450
Korpimäki, E. & Sulkava, S. 1987. Diet and
breeding performance of Ural owls Strix
uralensis under uctuating food condi-
tions. Ornis Fenn. 64: 57-66.
Lundberg, A. 1980. Vocalizations and court-
ship feeding of the Ural Owl Strix uralen-
sis. Ornis Scandinavica. 11: 65-70.
Lundberg, A. 1981: Population ecology of
the Ural Owl Strix uralensis in central
Sweden, Ornis Scandinavica. 12: 111-119.
Mikkola, H. 1983: Owls of Europe. Poyser,
Calton.
Norberg, R. Å. 1970. Hunting technique of
Tengmalm's owl Aegolius funereus. Ornis
Scandinavica. 1: 51-64.
Norberg, R. Å. 1978. Skull asymmetry, ear
structure and function, and auditory
localization in Tengmalm's owl, Aegolius
funereus (Linné). Philosophical Transactions
Royal Society London B 282: 325-410.
R Development Core Team 2017. R: A lan-
guage and environment for statistical
computing. R Foundation for Statistical
Computing, Vienna, Austria.
Slagsvold, T. & Sonerud, G. A. 2007. Prey
size and ingestion rate in raptors: impor-
tance for sex roles and reversed sexual size
dimorphism. Journal of Avian Biology 38:
650-661.
Slagsvold, T., Sonerud, G. A., Grønlien, H. E.
& Stige, L. C. 2010. Prey handling in rap-
tors in relation to their morphology and
feeding niches. Journal of Avian Biology
41: 488-497.
Solheim, R. 1985. Er slagugla truet i Norge?
Foreløpig rapport fra slagugleregistre-
ringer i perioden 1980-1984. Vår Fugle-
fauna 8: 19-22. (In Norwegian with
English summary).
Solheim, R. & Bjørnstad, R. 1987. Distribu-
tion and breeding biology in a marginal
population of Ural Owl Strix uralensis in
south- east Norway. Acta Reg. Soc. Sci.
Litt. Gothoburgensis 14:71-75.
Solheim, R., Bekken, J., Bjørnstad, R., Bye,
F.N., Hagen, T.K., Isaksen, K. & Strøm,
H. 2009. Ural Owls Strix uralensis at the
border line: nesting places are not a lim-
iting factor. In: Johnson D.H., Van Nieu-
wenhuyse D. & Duncan J.R. (eds.) Proc.
Fourth World Owl Conference Oct–Nov
2007, Groningen, The Netherlands. Ardea
97: 515–518.
Sonerud, G.A., Solheim, R. & Jacobsen, B.V.
1986. Home-range use and habitat selec-
tion during hunting in a male Tengmalm's
Owl Aegolius funereus. Fauna norvegica,
Ser. C, Cinclus 9:100-106.
Strøm, H. & Sonerud, G. A. 2001. Home
range and habitat selection in the Pygmy
Owl Glaucidium passerinum. Ornis Fen-
nica 78:145-158.
Valkama, J., Saurola, P., Lehikoinen, A., Lehi-
koinen, E., Piha, M., Sola, P. & Velmala,
W. 2014. The Finnish Bird Ringing Atlas.
Vol. II. Finnish Museum of Natural His-
tory and Ministry of Environment, Hel-
sinki.
Wolton, B. J. 1989. Kernel methods for esti-
mating the utilization distribution in home
range studies. Ecology 70:164-168.