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Introduction
Unsuitable forest management is a widespread and
common threat facing many forest bird species. Forestry
practices modify the original structure of most European
forests and promote changes in tree age and species com-
position [1, 2]. The changes in vegetation structure and
resource availability have a major impact on wildlife popu-
lations [3]. The present forestry reduces the availability of
old trees, dead wood and tree cavities [4]. Owls breeding in
woodlands are especially vulnerable because of their low
numbers and specific habitat requirements. Recently in
Poland, the introduction of reduced impact logging in for-
est management systems was proposed in some areas. This
system is based on retention of cavity-bearing trees and
selective harvest in forestry practice. The implementation
of this programme led to management more suitable for
hole-nesting forest birds, because it focused on the conser-
vation of old trees with nest-holes and provisioning of nest
boxes (also for owls). Unfortunately, in most of cases this
practice can be unsatisfactory in owl protection, because
most owl species need larger holes in natural areas with
good habitat conditions.
We analyzed tree age composition changes and
observed that the proportion of stands over 80 years old
increased during the study period. Therefore, we may
expect that the above-described changes can significantly
improve habitat conditions for the tawny owl in the study
area. In this analysis, we also control the impact of winter
conditions and potential food resources on the population in
our study area.
Tawny owls breed throughout most of Europe [5-6].
This species is the most common owl in Poland [7], but
there has been a lack of long-term studies on changes in the
Polish J. of Environ. Stud. Vol. 19, No. 5 (2010), 1039-1043
Original Research
The Role of Forest Age, Habitat Quality,
Food Resources and Weather Conditions
for Tawny Owl Strix aluco Populations
Jarosław Wiącek1*, Marcin Polak1, Grzegorz Grzywaczewski2
1Department of Nature Conservation, Maria Curie-Skłodowska University,
Akademicka 19, 20-033 Lublin, Poland,
2Department of Zoology, University of Life Sciences in Lublin,
Akademicka 13, 20-950 Lublin, Poland
Received: 27 November 2009
Accepted: 12 April 2010
Abstract
The main goal of this study was to determine the reason for the significant increase of the tawny owl
Strix aluco population in Kozłówka Forest near Lubartów (51°30' N, 22°35' E) in eastern Poland. Since 1990,
this forest complex has been part of Kozłowiecki Landscape Park. Our research was conducted using standard
playback method on a sample plot covering 50 km2. Each year in March and April 1990-91 and 2007-09, three
counts with vocal stimulation were performed on the study plot. Between 1991 and 2009, a significant increase
in the density of the tawny owl population was observed from 2.4 pairs/10 km2to 4.6 pairs/10 km2. We dis-
cuss how habitat quality, food availability, and weather conditions can explain this phenomenon.
Keywords:tawny owl, Strix aluco, population trends, owl conservation
*e-mail: wiacek@hektor.umcs.lublin.pl
abundance of this species [8]. The breeding density of
tawny owl populations has been studied in different habitats
in Poland [9]. It is common mostly in deciduous and mixed
old forests, reaching densities of 10-20 pairs/10 km2. In
coniferous forests and agricultural landscapes its density is
1-10 pairs/10 km2. Its total number in Poland is estimated to
be 65,000-75,000 pairs [10]. Despite this, knowledge about
the number and distribution of this species in Poland is not
sufficient, especially in respect to long-term changes in
numbers.
The aim of this study was to determine reasons for the
increased number of the tawny owl population in Kozłówka
Forest and to understand how the tawny owl responded to
the aging of forests and improving quality of habitats,
including weather conditions during the time of study.
Experimental Procedures
Study Area and Forest Management Changes
We examined the effects of aging forests, rodent cycle
stages (based on Quercus sp. seeds productivity), habitat
quality and weather conditions (temperature from
December to February and days with snow fall) on the
entire number of tawny owls in Kozłówka Forest near
Lubartów (51°30' N; 22°35' E) in eastern Poland. This for-
est complex was part of the Zamojscy estate. The surface of
the study area was 41 km2in 1990 and 50 km2in the fol-
lowing study seasons (1991 and 2007-09). The data from
1990 were excluded from further analyses because they had
a preliminary character and were obtained from a smaller
plot. The dominant form was pine (78%) and oak (13%)
stands. Habitats of the study area consisted of pine forest
(70%), hornbeam (20%) and alder (10%) [11]. Generally,
this proportion did not change between the 1991 and 2007-
09 seasons. We used information based on the number of
breeding territories gathered in 1991 and 2007-09. In the
last three seasons the studies were conducted based on the
high vole and mice number conditions (unpublished data
from State Forest Inspectorates Lubartów). Between these
two periods, the forest and owls conservation procedures
changed significantly. In order to preserve this forest com-
plex, the Kozłowiecki Landscape Park was established in
1990. During the last two decades, the age structure of the
forest changed. The share of the old-growth stands (81-120
years old) in the study plot increased from about 24% to
35% (Fig. 1). In 2001, twelve nest boxes for tawny owls
were placed on the study area by forest managers. All nest
boxes were checked once in April 2007 to find signs of use
by owls such as pellets, feathers, and prey remains, with
adult owls or eggs.
Sampling Methods
The sampling methods were the same in 1991 and
2007-09. We visited the entire sample plot three times dur-
ing one breeding season (February, March, and April) using
a nocturnal vocal stimulation with standard playback tech-
nique [12, 13]. During each visit, one to five groups of
observers were present in the field. Playback sessions of the
male “hoot” calls lasted 6 minutes (3 minutes of broadcast-
ing and 3 minutes of listening). Playback stations were
located at approximately 200 m intervals along the transect
routes. During all seasons, the same kind of male territorial
call was used. All observations were noted on official
forestry planning maps at 1:25,000 scale. Observations
were conducted in stable weather without strong winds and
rainfall. Meteorological conditions were obtained from
www.tutiempo.net/en/ from Radawiec Meteorological
Station (N 51°13'; E 22°23') located 20 km from our study
area. The preference of owls for stands in six age classes
(Fig. 1) was tested using the chi-square test. We analyzed
the role of forest age when comparing the potential habitats
and the nesting places.
Results and Discussion
Between 1991 and 2009 a significant increase of densi-
ty in the tawny owl population was observed from 2.4
pairs/10 km2to 4.6 pairs/10 km2(Table 1). In 1991 there
were 12 pairs on the study plot. The highest number of pairs
during the study period was recorded in 2009 – totaling 23
pairs (Table 1). Changes in the number and distribution of
breeding pairs between 1991 and 2007 is presented in Fig.
2. Among the 12 boxes placed in the study area in 2000, 7
boxes (58.3%) were already used by tawny owls before
2007. However, in the 2007 breeding season only one nest
1040 Wiącek J., et al.
Fig. 1. A comparison of age forest classes between 1981-90 and
1991-2000 in Kozłówka Forest.
Year Study area
(km2)
Number
of pairs
Density
(pairs/10 km2)
1991 50 12 2.4
2007 50 22 4.4
2008 50 19 3.8
2009 50 23 4.6
Table 1. The number and density of tawny owls in Kozłówka
Forest in 1991 and 2007-09.
box was used for breeding. In the following two seasons
none of the boxes were used. The analysis of preferences to
different age forest classes has shown a significant prefer-
ence of this species for older stands (χ2=105.3, df=5,
P<0.0001). In both study periods (1991 and 2007) most ter-
ritories were located in the oldest stands (>100 years) -
45%, following 27.5% for 81-100 years old and then
27.5%. for 61-80 years old. Changes of habitat selection
during the study period is presented in Table 2. In the
mature oak stands, the number of territories was stable dur-
ing the study period. However, we observed significant col-
onization of pine forests by tawny owls. The weather data
shows that in both study periods of 1991 and 2007-09,
weather conditions were similar. In both season, the snow-
falls were less intensive than during other winters (Fig. 3).
In these seasons, temperatures were higher in comparison
with typical Polish winter conditions (Fig. 4). These
weather traits were probably optimal for breeding owls.
A few seasons before the first study year, 1991, were poor
in oak seeds, while years prior to the second study period,
2007-09, were rich (Fig. 5). Seeds of oaks are the main food
for forest mice species such as Apodemus sylvaticus and A.
flavicollis, the main prey for the tawny in the study area
[31]. Oak masts are the main food of forest species of mice
Apodemus sp. and voles Microtus sp. Breeding density of
the tawny depends strongly on the density of small rodents
[17]. This data suggests that the first survey (1991) was
made in poor provisioning conditions for rodents, while the
following surveys (2007-09) were made during good food
conditions. Therefore, at the beginning of the 1990s, food
availability for owls might have been limited, while in the
last seasons (2007-09) availability of prey was rather high.
The Role of Forest Age, Habitat Quality, Food Resources... 1041
Habitat % habitat
stands
Number of
territories in
1991 (%)
Number of
territories in
2007 (%)
Pine forest 70 6 (50) 16 (72)
Hornbeam 20 6 (50) 5 (23)
Alder 10 01 (5)
Table 2. The comparison of the habitat selection between 1991
and 2007 in Kozłówka Forest.
Fig. 2. Number and distribution of tawny owl territories in 1991
and 2007 in Kozłówka Forest.
1991
2007
0
10
20
30
40
50
60
70
1991 2007
days
Fig. 3. Days with snowfall as cumulated index for December,
January, and February (1981-2008).
-30
-25
-20
-15
-10
-5
0
5
10
1990 2007
temperature ºC
Fig. 4. Mean temperature as cumulated index for December,
January, and February (1981-2008).
0
2,000
4,000
6,000
8,000
10,000
seeds of Quercus (kg)
1993 1997 2001 2007
Fig. 5. The seasons with high seed production (Quercus sp.) in
Kozłówka Forest.
The main result of this study was to verify the significant
increase in the tawny owl population. The Polish population
of this species increased in cities since the 1960s [14]. The
following data gathered in the 1970s and 1980s confirmed
the stability of this trend in the density of the tawny in large
towns [15, 16]. Lack of data from forest territories in Polish
literature from recent years makes the estimation of trends in
the forest habitat problematic. The only data for comparative
analyses of changes or trends in the breeding density of the
tawny in this habitat comes from our study. Significant
increase of the number of tawny owls in the forests of
Central Europe was reported by Hagemeijer and Blair [7].
Much data from France, Belgium, Luxembourg,
Netherlands, Germany, and Finland indicates an increase in
the number of breeding pairs [17]. Some rudimentary infor-
mation about changes in breeding density of this owl in east-
ern Poland can affirm that the tawny population significant-
ly increased [18, 19].
The density of the tawny as compared to other raptors
depends on food abundance and cycles in populations of
their prey. Many authors have reported strong correlation
between the breeding density of the Tawny and the num-
bers of their prey [6, 17]. Fluctuations between the number
of breeding pairs in the following years increased by 30%
as a consequence of prey availability [20], but the increase
recorded in our population was much higher than this value
and cannot be explained only by small mammals’ regular
cycles. Polish ornithological data confirm that the year
1991 was good for other raptors such as the short-eared owl
Asio flammeus and other predators feeding on small mam-
mals like mice or voles [21, 20], similar to 2007-09, when
our research in Kozłówka Forest was repeated (forest
inspectorate data).
The breeding density of the tawny owls from Kozłówka
Forest in 1991 was 2.4 pairs/10 km2. This data is situated as
a mean value for pine forests in Poland where breeding den-
sities in the 1980s changed from 2 pairs/10 km2[22] to 5
pairs/10 km2[23]. However, data from 2009, when the
breeding density was 4.6 pairs/10 km2, showed that nesting
conditions for this owl improved significantly. The age of
forest stands is important for the nesting of the tawny in
Kozłówka (Fig. 1). This positive change in the environment
caused an increase in the density this species of owl in the
study area. Most published data underline a strong relation
between breeding density and the age of forest stands in the
tawny owl habitat [5, 6, 17, 23, 24]. Additionally, providing
nest-boxes can help with strong colonization, in young,
suboptimal pine habitats in the study area. Such a situation
was described by Gramsz et al. [8]. Nest boxes placed in
comparatively young forest stands can make possible nest-
ing of the tawny owl. Unfortunately, nearly half of the nest
boxes in the study area were unsettled because they were
located near optimal habitats with natural holes. Natural
holes are probably more attractive for birds, therefore most
boxes near occupied territories were empty. The strong ter-
ritoriality of the tawny made it impossible to colonize these
boxes by other birds. Changing their location to far from
occupied territories makes possible settling by new pairs. On
the other hand, some authors underlined that too great a
number of nest boxes for the tawny owl increase their neg-
ative impact on other rare species of owl, such as the
pygmy owl Glaucidium passerinum, the little owl Athene
noctua or Tengmalm’s owl Aegolius funereus [8]. Some
data confirmed that the Tawny is increasing its density, dis-
placing the barn owl Tyto alba in the agricultural landscape
of eastern Poland [19].
In 1991 and 2007-09, a quick increase in the number of
territories in the pine forest was recorded. But the number
of the hornbeam territories was stable. In the situation of
low (1991) and high (2007-09) densities in the following
seasons, all available hornbeam territories were occupied
(5-6). New pairs came to settle only in pine or alder wood.
Therefore, strong growth in the numbers of territories was
only observed in the pine forest. This preference in order of
settlement in a different habitat was described in literature.
Tawny owls nest in different habitats but prefer old, broad-
leafed forest [25]. Therefore, breeding densities recorded in
hornbeam habitats (usually above 5 pairs/10 km2) were sig-
nificantly higher in comparison with habitats in pine woods
[11, 26-30].
The age of tree stands in the study area significantly
changed between the 1991 and 2007-09 seasons. But a
strong increase in the number of pairs probably had an
effect on other factors too. Many authors underlined that
weather conditions can influence the breeding density in
many owl species [6]. Data from Poland (Białowieski
National Park) suggest that mild winters with small snow-
fall can help with the survival of small mammals and their
detection by raptors [20]. During the time of our study in
Kozłówka Forest, the tawny population was monitored in
the comparatively mild winters in comparison to, in gener-
al, harsh Polish winter conditions. Additionally, a small
number of days with snow fall was observed in the study
seasons. These factors can help with high hunting success
and high density breeding in both study seasons. However,
a much lower breeding density observed in the 1991 season
was the effect of strong winter conditions in former years
(1985-87) when the tawny population probably decreased
strongly (Figs. 3 and 4). This phenomenon was compared
with tawny counts in Kozłówka forest in 2010, when after
a harsh, snowy and long winter we recorded their decline in
a number of territories (our unpublished data).
The present forest management in Poland is based on
protection of old trees with natural holes. Theoretically, this
system can favour highly territorial and sedentary species
such as the tawny. A few old trees with holes remaining in
the clearing areas can provide a safe nesting place for this
owl. In practice, the reduced impact of logging was proba-
bly not a main reason for the strong increase of pair num-
bers.
Conclusions
In summary, the number of breeding pairs of the tawny
owl in the study area significantly increased since the
1990s. The main reasons for this increase were the aging of
the forest, connected with improvement of habitat quality.
The number of breeding pairs depended on good weather
1042 Wiącek J., et al.
conditions and food abundance. The presence of nest-boxes
and the reduced impact of logging in forest practices cannot
significantly help in density increase of the tawny owl in
the study area. We need data from the plots located in other
regions of Poland to determine the true trend in Tawny Owl
population changes throughout the country.
Acknowledgements
We are grateful to Beata Niedźwiedź, Diana
Płudowska, Kalina Łapińska, Paweł Łapiński, Małgorzata
Patyniak, Waldemar Gustaw, Piotr Karpiński, Magdalena
Małkowska, and other students from Maria Curie-
Skłodowska University and the University of Life Sciences
in Lublin and State Forest Inspectorates Lubartów for their
help during the fieldwork.
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