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Nest-site selection and nesting success of Little Owls (Athene noctua) in Mediterranean woodland and open habitats

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We studied nest-site selection by Little Owls (Athene noctua) in two Mediterranean habitats, a holm oak (Quercus rotundifolia) woodland (36 nests during 1997-99) and a steppe-like area (37 nests during 1997-99), in southern Portugal, by comparing macrohabitat and microhabitat characteristics of used nests to those of randomly-selected nest sites. In the woodland area, predator presence seemed to be the main factor that influenced nest-site selection by owls. In the steppe area, a large number of alternative cavities around a nest appeared as the most important variable associated with nest-site selection. At this site, size of stones in stone piles also seemed to influence nest-site selection; owls nested in piles with larger than average stones. We also found tree girth was positively associated with nesting success in the woodland area. Larger trees held more complex cavities that may have improved the ability of adults and offspring in escaping from predators. Other variables, such as distance to human habitations and the orientation of nest-site entrance might have influenced nesting success as well. Predation risk and the internal features of nest cavities were the most likely factors affecting nest-site selection and nesting success of Little Owls in Mediterranean habitats.
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RAPTOR RESEARCH
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35
J. Raptor Res. 38(1):35–46
q
2004 The Raptor Research Foundation, Inc.
NEST-SITE SELECTION AND NESTING SUCCESS OF LITTLE OWLS
(ATHENE NOCTUA) IN MEDITERRANEAN WOODLAND AND
OPEN HABITATS
R
ICARDO
T
OME
´
1
Section of Ecology, Department of Biology, University of Turku, FIN 20014 Turku, Finland
C
AROLINA
B
LOISE
Rua Barbosa du Bocage, 31, Lourel, 2710 Sintra, Portugal
E
RKKI
K
ORPIMA
¨KI
Section of Ecology, Department of Biology, University of Turku, FIN 20014 Turku, Finland
A
BSTRACT
.—We studied nest-site selection by Little Owls (Athene noctua) in two Mediterranean habitats,
a holm oak (Quercus rotundifolia) woodland (36 nests during 1997–99) and a steppe-like area (37 nests
during 1997–99), in southern Portugal, by comparing macrohabitat and microhabitat characteristics of
used nests to those of randomly-selected nest sites. In the woodland area, predator presence seemed to
be the main factor that influenced nest-site selection by owls. In the steppe area, a large number of
alternative cavities around a nest appeared as the most important variable associated with nest-site
selection. At this site, size of stones in stone piles also seemed to influence nest-site selection; owls
nested in piles with larger than average stones. We also found tree girth was positively associated with
nesting success in the woodland area. Larger trees held more complex cavities that may have improved
the ability of adults and offspring in escaping from predators. Other variables, such as distance to human
habitations and the orientation of nest-site entrance might have influenced nesting success as well.
Predation risk and the internal features of nest cavities were the most likely factors affecting nest-site
selection and nesting success of Little Owls in Mediterranean habitats.
K
EY
W
ORDS
:Little Owl; Athene noctua; habitat selection;Mediterranean region;predation risk;reproductive
success.
SELECCIO
´N DE SITIO-NIDO Y E
´XITO EN LA ANIDACIO
´N DE LOS BUHO
´S PEQUEN
˜OS (ATHENE
NOCTUA) EN BOSQUES Y HABITATS ABIERTOS DEL MEDITERRA
´NEO
R
ESUMEN
.—Estudiamos la seleccio´n de sitios nido para los bu´hos pequen˜os (Athene noctua) en dos ha´b-
itats del mediterra´neo, un bosque de roble acebo (Quercus rotundifolia), con 36 nidos durante 1997—
99,yuna´rea parecida a una estepa (37 nidos durante 1997—99) en el sur de Portugal, para comparar
las caracterı´sticas de uso de los nidos en macro y micro ha´bitats en aquellos sitios nido seleccionados
aleatoreamente. En el a´rea boscosa, la presencia de depredadores parece ser el principal factor que
influyo´ en la seleccio´ n de sitios nido por parte de los bu´ hos. En el a´ rea de estepa, un gran numero de
cavidades alternativas alrededor del nido parece ser la variable mas importante asociada con la seleccio´n
del sitio-nido. En este sitio, el taman˜ o de las rocas en la pila de piedras parece influenciar la seleccio´n
de los sitios nido; los bu´ hos anidaron en pilas con piedras mas grandes que el promedio. Adema´s
encontramos tres circunferencias que se asociaron positivamente con el e´ xito en la anidacio´ n en el a´ rea
boscosa. Los a´rboles ma´s grandes proveen mayor complejidad que puede haber mejorado la habilidad
de los adultos y su prole para escapar de los depredadores. Otras variables, tales como la distancia a las
habitaciones humanas y la orientacio´ n de la entrada de los sitios-nido pudieron igualmente haber in-
fluenciado el e´xito en la anidacio´ n. Los riesgos de depredacio´ n y las caracterı´sticas internas de las
cavidades de los nidos probablemente fueron los factores que ma´s afectaron la seleccio´ n de sitios nido
yele´xito en la anidacio´ n de los bu´ hos pequen˜os en los ha´bitats del Mediterra´ neo.
[Traduccio´ n de Ce´ sar Ma´rquez]
1
E-mail address: ricmocho@iol.pt
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Nest-site selection is a key component of habitat
selection by birds (Hilde´n 1965), with important
consequences for survival and reproduction of in-
dividuals (Cody 1985). Nest predation is a major
cause of reproductive loss in birds and is often con-
sidered as a strong selective force in the evolution
of nesting and dispersal strategies (e.g., Newton
1979, Martin 1992, 1995, Hakkarainen et al. 2001).
To reduce the risk of nest predation and losses due
to adverse weather, birds have adopted strategies
such as cavity nesting (e.g., von Haartman 1957).
Consequently, cavity nesters often have higher
breeding success than open-nesting species (e.g.,
Lack 1954, Nice 1957, Peterson and Gauthier 1985,
Korpima¨ki 1987).
Nevertheless, high nest predation rates have also
been recorded in some hole-nesting species (e.g.,
Flegg and Cox 1975, Dunn 1977, Eriksson 1979,
Sonerud 1985b), and thus additional tactics to
minimize nest predation and increase offspring
production might be expected to influence nest-
site selection of cavity-nesting birds. Nest-site vari-
ables, such as cavity dimensions, volume, height,
and depth might be important (e.g., Stauffer and
Best 1982, Van Balen et al. 1982, Peterson and Gau-
thier 1985, Belthoff and Ritchison 1990) and influ-
ence reproductive success (e.g., Karlsson and Nils-
son 1977, Nilsson 1984, Korpima¨ki 1985, Rendell
and Robertson 1989, Valkama and Korpima¨ki
1999). Microhabitat variables, such as tree species
and density (Swallow et al. 1986) or the vegetation
surrounding the cavity (McCallum and Gehlbach
1988, Valkama et al. 1995, Valkama and Korpima¨ki
1999) may also affect nest-site selection.
The Little Owl (Athene noctua) is a small owl
mainly associated with farmland and open wood-
land habitats, where it breeds mostly in holes in
trees, but it also uses cavities in stone piles and
buildings, or even holes in the ground (Scho¨nn et
al. 1991, Ge´ not and Van Nieuwenhuyse 2002).
Over the last decades, Little Owl populations have
declined severely throughout most of Europe, and
the species is now listed as a ‘‘SPEC 3’’ species (i.e.,
a species whose global populations are not concen-
trated in Europe, but which have an unfavourable
conservation status in Europe; Tucker and Heath
1994). This decrease have been caused by habitat
changes due to intensification of agriculture, in-
cluding elimination of nest sites, a decrease in prey
abundance, and detrimental effects of pesticides
on breeding success (Scho¨nn et al. 1991, Exo 1992,
Tucker and Heath 1994, Ge´not and Van Nieuwen-
huyse 2002).
Although several studies have described nest
sites used by Little Owls, very few authors paid at-
tention to nest-site selection strategies (Ge´not and
Van Nieuwenhuyse 2002), particularly in steppe-
like habitats. To our knowledge, only Juillard et al.
(1992) and Centili (2001) reported on nest char-
acteristics in steppe habitats, but they did not com-
pare used nest-sites with available cavities, and thus
provided no information on nest-site selection.
In western and central European farmland,
where mechanization and intensification of agri-
culture has led to the scarcity of nest-sites, erection
of nest-boxes has been adopted successfully to in-
crease or maintain local populations of Little Owls
(Kneule and Michels 1994, Bultot et al. 2001).
However, in Mediterranean habitats of southern
Europe, where the species is still relatively abun-
dant, habitat features, and particularly nest sites,
may be managed adequately for conservation.
Therefore, our aim was to investigate features
linked to nest site selection by Little Owls in two
different types of Mediterranean habitat. With this
study, we collected data to develop management
guidelines for the conservation of Little Owls. Fur-
thermore, because individuals should prefer nest-
site features that increase reproductive success
(Alatalo et al. 1984, Leonard and Picman 1987,
Milks and Picman 1994), we also examined rela-
tionships between nest-site variables and nesting
success of owls.
M
ETHODS
Study Areas. The study was conducted in two areas lo-
cated approximately 22 km apart, in the Baixo Alentejo
province, Southern Portugal: Cabec¸ a da Serra (37
8
37
9
N,
8
8
09
9
W) and S. Marcos da Atabueira (37
8
42
9
N, 7
8
50’W).
Cabec¸ a da Serra comprised 5.6 km
2
of very open old
holm oak (Quercus rotundifolia) woodland. The area is
used as pasture for cattle or cereal cultivation and a small
part is covered by a young plantation of stone pine (Pinus
pinea). The density of Little Owls in this area is very high,
with ca. 7 pairs/km
2
(R. Tome´ unpubl. data). S. Marcos
da Atabueira is a steppe-like area of 15.7 km
2
and is also
used for cattle pasture and cereal cultivation. Trees are
absent, with the exception of a small (
,
0.3 km
2
) plan-
tation of blue gum (Eucalyptus globulus). Most of the area
is managed for nature conservation. Little Owl popula-
tion is less dense than in Cabec¸ a da Serra, with ca. 2.3
pairs/km
2
(R. Tome´ unpubl. data).
Nest-Sites and Random Cavities. We searched for nests
of Little Owls during the breeding seasons of 1997–99.
Nest sites were mainly located by following male owls tak-
ing prey to incubating or brooding females, or young. In
other cases, we detected nests by checking cavities in
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places where adult birds were often observed. We mea-
sured several features of each nest site, including char-
acteristics of the surrounding habitat. Depending on
whether the nest was located in a tree or in a stone pile,
the two main nesting environments for Little Owls in the
study areas, we identified the tree species and measured
its diameter at 1.40 m (diameter at breast height, DBH),
tree height, and stone pile height, length, and width.
Type of stone pile (‘‘tower,’’ ‘‘collapsing tower,’’ ‘‘wall,’
hide for hunters, simple aggregation of stones, collapsed
pile, and pile partially buried on the ground) and stabil-
ity (two classes: stable, if the pile could support a person
without threatening collapse; unstable, if it could not)
were also classified. Mean dimensions (long and short
diameters) of stones composing stone piles were deter-
mined by measuring ten stones. For nest-site entrances,
we measured height (distance to the ground), long and
short diameters, inclination (in degrees, corresponding
0
8
to a horizontal entrance and 90
8
to a vertical one) and
orientation. For nests in trees, coverage (percent of the
entrance covered by leaves or branches when viewed
from 10 m, from the direction of the entrance) and site
(trunk, base of branch, branch) in the tree were deter-
mined. For nest sites with more than one entrance hole,
we measured the hole most often used (in all cases only
one entrance hole was observed to be used). We also
checked for the existence of alternative entrance holes
to the same nest and for the presence of potential pred-
ator sign (e.g., feces of mustelids, foxes, rats or jewelled
lizards [Lacerta lepida]).
We measured additional features within a 100-m radius
of the nest site. We recorded the number and type of
available perches and the number and type of available
nesting cavities. We considered as perches any structure
with a minimum height of 50 cm, because owls often
hunted from perches this low. Each tree or stone pile was
considered as one perch, irrespective of the number of
possible individual perch sites (for instance, branches) it
contained. Potential nesting cavities were defined as hav-
ing a minimum depth of 50 cm and dimensions that ap-
peared large enough for Little Owls (i.e., an opening
greater than 8 cm in diameter; see also Juillard 1980, Exo
1981, Ge´not 1990 and Belthoff, and Ritchison 1990 for
the Eastern Screech Owl [Otus asio]). A tree or stone pile
with at least one suitable cavity was considered as one
available nesting site even if it had additional cavities.
Other habitat features that were recorded included type
of habitat and distance from the nest to the nearest road,
pathway, human habitation, reservoir, and permanent
stream. Distance measurements were made with the help
of aerial photographs (1:15 000). Sample sizes of the var-
iables were not equal, because it was impossible to mea-
sure all characteristics of some nests (for example, we
could not be sure about the nest entrance, in some cases,
in stone piles).
In the woodland area, 36 nests were found in 26 ter-
ritories during the study period. Three different nests
were used in one territor y and two different nests were
used in seven territories. In the steppe area, 37 nests were
found in 30 territories. Two different nests were used in
seven territories throughout the study. As in some other
studies (e.g., Belthoff and Ritchison 1990, Sedgwick and
Knopf 1990), we included all the different nests found,
because at least one of the parent owls changed in most
of the territories during the study period (R. Tome´ un-
publ. data).
We selected 22 locations randomly for each study area
and plotted these on a map with a numbered grid (e.g.,
Titus and Mosher 1981). Once random points were lo-
cated in the field, the nearest available nesting cavity
(same criteria as above) that was unused during the study
period was chosen for comparison with occupied nests,
and the same measurements taken.
Nesting Success. We recorded the success or failure of
each nesting attempt whenever possible. This variable
was 1 in cases where at least one juvenile fledged, or 0
in cases of no fledglings. Due to the depth and shape of
the nest cavities, it was difficult to confirm the cause of
failure in many cases (Glue and Scott 1980). Failure was
ascribed to predation in cases where eggs disappeared or
when we found destroyed eggs or the remains of nest-
lings.
Data Analysis. We compared nest-site variables between
used and random cavities using parametric or nonpara-
metric tests. Likewise, we compared variables in success-
ful and unsuccessful nests. In three cases (one in the
woodland area and two in the steppe area), nest sites
were included in both categories, because they were suc-
cessful and unsuccessful in different years. Continuous
variables that met assumptions of homoscedasticity and
normality were compared using t-tests. Variables that vi-
olated the assumptions were log
e
- or square-root trans-
formed prior to analysis (Sokal and Rohlf 1981, Zar
1996), or were analyzed using Mann-Whitney U-tests. For
categorical variables, contingency analyses were used to
compare relative frequencies of used nests versus random
cavities (Zar 1996). Mean cavity-entrance orientation (a¯
6
angular deviation) and its dispersion (r) were calcu-
lated for both used nests and random cavities, and Ray-
leigh’s tests (Zar 1996) were used to determine if a sig-
nificant mean population existed in either sample.
Differences in mean directions of entrance holes be-
tween used nests and unused cavities were examined us-
ing the nonparametric Watson’s test (Zar 1996).
We used logistic regression to evaluate simultaneously
the effect of different variables and their interactions on
the nest-site selection and then on nesting success. These
analyses treated the dependent variables as binary re-
sponse variables (1
5
used nest, 0
5
random point and
1
5
successful nest, 0
5
failed nest, respectively). All var-
iables that had a univariate P-value
,
0.25 were entered
in the initial multivariate model, together with their first-
order interactions (Hosmer and Lemeshow 1989). Non-
significant interactions and main effects were gradually
removed from the model, starting from the least signifi-
cant variable. In this way, only significant main effects
and interactions were included in the final model (Chris-
tensen 1990, Tremblay et al. 1997, Valkama et al. 1998).
We decided not to pool data from both study areas in
the analyses, because the different nature of tree and
stone pile cavities resulted in basic scale and categorical
differences among most variables measured. Data were
analysed using SPSS statistical package (Norusis 1993).
R
ESULTS
Nest-Site Characteristics and Nest-Site Selection.
Thirty-three out of the 36 nests in the woodland
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Table 1. Characteristics measured in nests and random sites in the woodland area. Variables in bold with univariate
P-values
,
0.25 were included in the full logistic regression model. DBH is diameter at breast height.
N
ESTS
M
EAN
6
SE N
R
ANDOM
S
ITES
M
EAN
6
SE N
U
NIVARIATE
P-
VALUE
Tree
DBH (m)
Height (m)
0.67
6
0.16
6.33
6
1.98
30
30
0.74
6
0.19
6.36
6
1.89
21
21
0.125
0.946
Entrance
Height (m)
Long diameter (cm)
Short diameter (cm)
Inclination (
8
)
Cover (%)
1.94
6
0.85
21.70
6
12.63
15.15
6
9.00
52.03
6
28.31
8.75
6
26.70
34
33
33
32
32
1.79
6
1.13
19.14
6
11.01
12.20
6
7.07
56.59
6
34.55
3.10
6
13.08
22
22
22
22
21
0.700
0.399
0.177
0.597
0.487
Surroundings (100-m radius)
No. of perches
No. of cavities
Trees/ha
42.42
6
40.07
4.56
6
3.17
4.34
6
5.09
36
36
36
29.95
6
24.25
3.18
6
2.36
3.45
6
3.94
22
22
22
0.210
0.074
0.163
Distances to
Road
Pathway (m)
Human
a
(m)
Reservoir (m)
Stream (m)
821.39
6
832.90
114.15
6
114.21
805.25
6
452.19
790.47
6
483.79
413.50
6
240.50
36
36
36
36
36
849.95
6
810.89
109.73
6
91.11
957.27
6
449.85
757.55
6
473.95
399.59
6
266.60
22
22
22
22
22
0.804
0.972
0.195
0.903
0.838
a
Distance from the nest to the nearest human habitation.
area were located in trees. Two were in stone piles
and one in a hole under a road. Entrance holes
were located on average
,
2 m above the ground
(Table 1). None of the continuous variables dif-
fered significantly between nests and randomly-se-
lected unused cavities, although six variables had
univariate P-values less than 0.25 and were entered
in the initial logistic regression model (Table 1).
Mean entrance orientation was 162
86
73.1 (r
5
0.19) for the nests and 175
86
72.2 (r
5
0.21) for
the random cavities (Fig. 1a). None of the popu-
lations showed significant directionality (Rayleigh’s
test: Z
5
1.16, P
.
0.20 for nests; Z
5
0.90, P
.
0.20 for random sites) and there was no significant
difference between the mean entrance orientation
of nests and random cavities (U
2
5
0.01, P
.
0.5).
Proportions of nests and random cavities did not
differ significantly relative to the tree species, num-
ber of entrance holes, place of the entrance holes
and habitat (Table 2). However, the proportion of
nests where some predator signs were found was
significantly smaller than at random sites (Table 2)
and this variable was included in the initial logistic
regression model.
In the steppe area, 36 nests were located in stone
piles and one in a hole in a wall. Used stone piles
were on average relatively large, although low, and
the nest entrance was usually situated
,
0.5 m
above the ground (Table 3). Mean stone dimen-
sions were significantly larger in stone piles used
for nests than in unused piles (long stone diame-
ter: t
5
3.09, P
5
0.003; short stone diameter: t
5
2.85, P
5
0.006; N
5
36 nests, N
5
22 unused piles;
Table 3). Likewise, the number of additional suit-
able cavities around nests was also significantly
greater than around random sites (Mann-Whitney
U-test, z
52
2.57, P
5
0.01; Table 3). Nests were
also significantly closer to pathways than unused
piles (t
52
2.38, P
5
0.021; Table 3). Long stone
diameter (correlated with short stone diameter:
Spearman r
5
0.85, P
,
0.001), the number of
suitable cavities, and distance to pathways were en-
tered in the initial logistic regression model, to-
gether with small entrance diameter, which also
had a univariate P
,
0.25 (Table 3). In this area,
the mean entrance orientation was 34
86
78.1 (r
5
0.07) for nests and 253
86
65.6 (r
5
0.35) for
random cavities (Fig. 1b). None of the populations
showed significant directionality (Rayleigh’s test: Z
5
0.133, P
.
0.50 for nests; Z
5
2.50, P
.
0.05 for
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Figure 1. Cavity-entrance orientation of Little Owl nests
(solid arrows) and randomly selected cavities (dashed ar-
rows) in the (a) woodland and (b) steppe areas. Arrows
represent mean direction for each distribution and their
lengths correspond to the relative concentration (r; Zar
1996) of observations
random sites) and there was no significant differ-
ence between the mean entrance orientation of
nests and random cavities (U
2
5
0.09, P
.
0.20).
The proportions of nests and random cavities did
not differ significantly relative to the stone pile
type, stone pile stability, habitat and predator pres-
ence (Table 4). The proportion of nests with more
than one entrance hole was significantly greater
than for random sites. Four of these categorical
variables showed P-values
,
0.25 and were there-
fore included in the initial logistic regression mod-
el (Table 4).
Predator presence was the only variable to enter
in the final logistic regression that modelled char-
acteristics of nests and random sites in the wood-
land area (
x
2
5
4.44, df
5
1, P
5
0.035). In the
steppe area, the number of suitable nesting cavities
and the long diameter of stones in piles were the
two variables entering the final model (number of
cavities:
x
2
5
4.93, df
5
1, P
5
0.026; long stone
diameter:
x
2
5
3.62, df
5
1, P
5
0.057).
Nesting Success. Of all nests with known output
(46 nests), 26.1% failed in the woodland area. In
the steppe area, this proportion was 33.3% (N
5
45). Almost half (48%) of the failures could be
attributed to predation, although it is likely that
many of the remaining nest failures were due to
this factor as well (e.g., nests that were abandoned
during late phase of incubation or during the nest-
ling period).
Five continuous variables showed univariate P-
values lower than 0.25 when comparing differences
between successful and unsuccessful nests in the
woodland area (Table 5). In successful nests, DBH
was significantly larger and nests were significantly
further away from human habitation (Table 5).
Mean entrance orientation was 264
86
65.8 (r
5
0.34) for successful nests and 25
86
41.1 (r
5
0.74)
for nests that failed (Fig. 2). Successful nests did
not show significant directionality (Rayleigh’s test,
Z
5
2.79, P
.
0.05), but entrance orientation in
failed nests was significantly different from a ho-
mogeneous circular distribution (Rayleigh’s test, Z
5
5.52, P
,
0.002). Mean entrance orientation of
the two nest types differed significantly (U
2
5
0.46,
p
,
0.01). Habitat type was the only categorical
variable with univariate P-value lower than 0.25
when comparing successful and unsuccessful nests
(Likelihood Ratio
5
5.37, df
5
2, P
5
0.068, N
5
37).
In the steppe area, none of the measured vari-
ables showed significant differences between suc-
cessful and unsuccessful nests. Only stone pile
height (Mann-Whitney U-test, z
52
1.78, P
5
0.075, N
5
36) and type of stone pile (Fisher’s Ex-
act Test, P
5
0.089, N
5
36) showed univariate P-
values
,
0.25.
In the woodland area, only DBH was included
in the final logistic regression model comparing
the characteristics of successful and unsuccessful
nests (
x
2
5
3.98, df
5
1, P
5
0.046). Distance from
nests to nearest human habitation was positively
correlated with DBH (Pearson r
5
0.36, P
5
0.0495) and could be an alternative explanatory
variable in the final model (
x
2
5
5.85, df
5
1, P
5
0.016). In the steppe area, there were no signifi-
cant main effects in the final model.
40 V
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Table 2. Frequencies of categorical variables in nests and random sites in the woodland area. Differences were tested
using
x
2
tests and Fisher’s Exact Test. Variables in bold with univariate P-values
,
0.25 were included in the full
logistic regression model. Habitat categories considered were: pasture during 3 yr (study period), young plantation
during 3 yr, pasture during one part of the study, and cereal fields during another.
N
ESTS
(%)
R
ANDOM
S
ITES
(%) T
EST
P
Type of structure
Holm oak
Other
29 (80.6)
7 (19.4)
17 (77.3)
5 (22.7)
Fisher’s 0.75
Number of Entrances
1
.
1
28 (84.9)
5 (15.1)
19 (86.4)
3 (13.6)
Fisher’s 1.00
Entrance site
Trunk
Base of branches
Branches
9 (29.0)
14 (45.2)
8 (25.8)
9 (42.9)
5 (23.8)
7 (33.3)
x
2
5
2.50 0.29
Habitat
Pasture
Young plantation
Pasture/cereal
15 (41.7)
6 (16.6)
15 (41.7)
7 (31.8)
4 (18.2)
11 (50.0)
x
2
5
0.58 0.75
Predator
Absent
Present
25 (89.3)
3 (10.7)
7 (58.3)
5 (41.7)
Fisher’s 0.04*
* Significant, P
,
0.05.
D
ISCUSSION
Nest-Site Selection. According to our results,
predator presence emerged as the main factor
linked to nest-site selection by Little Owls in the
woodland area. Predation has also been reported
as one of the major factors affecting the breeding
success of this owl (Exo and Hennes 1980, Scho¨nn
1986), and the avoidance of predators was identi-
fied as one of the most important factors when se-
lecting a nesting site for other cavity-nesting spe-
cies (e.g., Rendell and Robertson 1989).
The main predators of Little Owl nests in our
study areas were mammals such as the stone mar-
ten (Martes foina), the common genet (Genetta ge-
netta), and the garden dormouse (Eliomys querci-
nus), as well as the jewelled lizard (Kno¨tzsch 1978,
Scho¨nn 1986, Juillard et al. 1992, Ge´ not 2001). All
these species seek shelter and roost in cavities, and
thus, include a number of cavities in their home
ranges. By not nesting in trees used by predators,
Little Owls probably reduce the chance of being
killed by a predator. An alternative hypothesis is
that owls use these sites, but are able to keep most
predators away from their nests. Little Owls may
attack predators near nests (Glutz and Bauer 1980,
Cramp 1985); however, our observations suggest
that, at least toward a human intruder, they gen-
erally limited their nest-defence actions to a few
alarm calls.
The large number of suitable natural cavities in
the holm oak woodland of our study area makes it
unlikely that nest-site availability was limiting the
breeding density of Little Owls (Exo 1983, Loske
1986, Dalbek et al. 1999). This conclusion was sup-
ported by the low use of 50 nest boxes that were
available in our 4-yr study. Only one box was oc-
cupied on three occasions, by a different owl pair.
Because the use of nest-boxes often indicates nest-
site limitation (e.g., Lundberg and Westman 1984,
Brawn and Balda 1988; Kno¨tzsch 1988); Exo 1992
for the Little Owl), this result supports the idea
that nest sites are not limiting in our study area
(Brush 1983), and that owls may be able to select
nest sites that are relatively safe from predators.
This suggestion was supported by our results on
the frequency of nesting failure and nest preda-
tion, which were relatively low when compared to
data from studies on the Little Owl in other parts
of Europe (Glue and Scott 1980, Exo 1983, Juillard
1984, Scho¨nn 1986, Ge´ not 2001).
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Table 3. Characteristics measured in nests and random sites in the steppe area. Variables in bold with univariate P-
values
,
0.25 were included in the full logistic regression model.
N
ESTS
M
EAN
6
SE N
R
ANDOM
S
ITES
M
EAN
6
SE N
U
NIVARIATE
P-
VALUE
Stone pile
Long Diameter (m)
Short Diameter (m)
Height (m)
8.66
6
11.06
3.92
6
2.00
1.19
6
0.74
36
36
36
6.86
6
5.34
4.40
6
2.76
1.14
6
0.51
22
22
22
0.569
0.641
0.798
Stone
Long diameter (cm) 31.52
6
9.19 35 25.08
6
6.66 21 0.003
Short diameter (cm) 19.85
6
5.44 35 16.10
6
3.84 21 0.006
Entrance
Height (m)
Long diameter (cm)
0.45
6
0.57
21.32
6
12.98
23
22
0.42
6
0.29
17.43
6
10.92
22
22
0.532
0.257
Short diameter (cm) 11.20
6
5.11 22 9.05
6
3.60 22 0.124
Inclination (
8
) 71.82
6
24.38 22 72.27
6
28.48 22 0.542
Surroundings (100-m radius)
No. of perches 25.84
6
32.12 37 22.18
6
28.81 22 0.446
No. of cavities 2.92
6
2.75 37 1.68
6
1.73 22 0.010
Distances to
Road (m) 1428.43
6
586.84 37 1333.77
6
533.50 22 0.471
Pathway (m) 141.68
6
123.58 37 222.14
6
128.54 22 0.021
Human
a
(m)
Reservoir (m)
Stream (m)
1157.65
6
404.89
835.24
6
363.79
704.81
6
515.11
37
37
37
1138.77
6
394.99
729.05
6
368.45
864.41
6
581.23
22
22
22
0.862
0.285
0.277
a
Distance from the nest to the nearest human habitation.
In the woodland area, none of the other nest-
site characteristics appeared to influence nest-site
selection by Little Owls. This should be interpreted
with caution; however, because it is possible that
‘‘internal’’ characteristics of cavities (such as
length and shape of access to the nest chamber or
dimensions) may influence nest-site selection.
Glue and Scott (1980) and Exo (1981) mentioned
that Little Owls used mainly deep chambers,
reached by long and winding passages (Scho¨nn et
al. 1991). These internal cavity dimensions were
not measured in our study. Moreover, the fact that
we restricted the selection of random cavities to
ones that seemed suitable for owls increased the
realism of the test, but reduced the likelihood of
finding significant differences.
In the steppe area, the number of alternative
suitable cavities emerged as the main variable ex-
plaining nest-site selection. Although Little Owls
often show strong nest-site fidelity (with individual
variation; Glue and Scott 1980, Glutz and Bauer
1980, Ullrich 1980, Exo 1981), they may benefit
from the inclusion of alternative suitable nesting
cavities in their territories. In many species of birds
( Jackson 1994, Marjakangas et al. 1997, Valkama et
al. 1998), including cavity-nesters (e.g., Eriksson
1979, Dow and Fredga 1983, Sonerud 1985, Hak-
karainen et al. 2001; but see Korpima¨ki 1987,
1993), individuals avoid breeding in sites where
they have failed in previous attempts due to pre-
dation, probably because predators may revisit
these sites. This could select for individuals that
shift nest holes between breeding attempts. The
inclusion of a large number of suitable cavities in
a territory may also allow Little Owls to switch to
alternative sites in the case of a stone pile collapse
due to erosion, and provide alternative roosting
places, both for adults and fledglings (Scho¨nn et
al. 1991, Short 1979, Sedgwick and Knopf 1990).
The number of potential cavities was much larger
in the woodland area than in the steppe area (on
average almost the double number of suitable cav-
ities) and probably decreased the importance of
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Table 4. Frequencies of categorical variables in nests and random sites in the steppe area. Differences were tested
using
x
2
tests and Fisher’s Exact Test. Variables in bold with univariate P-values
,
0.25 were included in the full
logistic regression model. Habitat categories considered were: pasture during 3 yr (study period) and used for agri-
cultural crops in at least 1 yr.
N
ESTS
(%)
R
ANDOM
S
ITES
(%) T
EST
P
Stone pile type
‘‘Built’’
Other
5 (13.9)
31 (86.1)
7 (33.3)
14 (66.7)
Fisher’s 0.11
Stone pile stability
Unstable
Stable
12 (42.9)
16 (57.1)
9 (42.9)
12 (57.1)
x
2
5
0 1.00
Number of Entrances
1
.
1
16 (80.0)
4 (20.0)
22 (100.0)
0 (0.0)
Fisher’s 0.04*
Habitat
Pasture
Used for crops
17 (46.0)
20 (54.0)
6 (27.3)
16 (72.7)
x
2
5
1.31 0.25
Predator
Absent
Present
7 (28.0)
18 (72.0)
7 (58.3)
5 (41.7)
Fisher’s 0.15
* Significant, P
,
0.05.
Table 5. Continuous variables that differed between successful and failed nests by univariate P-values
,
0.25 in the
woodland area, and were included in the full logistic regression model. Differences were tested using t-tests and
Mann-Whitney U-tests.
S
UCCESSFUL
N
ESTS
M
EAN
6
SE N
F
AILED
N
ESTS
M
EAN
6
SE NT
EST
P
Structure
Tree DBH3
a
(m) 0.71
6
0.16 21 0.58
6
0.01 10 t
5
2.87 0.008*
Surroundings (100-m radius)
Trees/ha 3.35
6
2.42 26 6.48
6
8.31 11 U
5
101.50 0.167
Distances
Road (m)
Human (m)
Reservoir (m)
982.65
6
843.53
926.85
6
448.35
676.19
6
438.01
26
26
26
613.73
6
947.85
478.73
6
275.48
1040.55
6
491.61
11
11
11
U
5
89.00
t
5
3.71
U
5
85.50
0.073
0.001*
0.056
a
DBH
5
Diameter at breast height.
* Significant, P
,
0.05.
this variable in the final model for the woodland
area.
According to our results, size of stones appeared
to be an additional factor influencing nest-site se-
lection in the steppe area, because stones in stone
piles used for nesting were larger than stones in
random sites. Larger stones probably create more
internal cavities within piles ( Juillard et al. 1992),
and also deeper cavities that owls usually prefer
(Glue and Scott 1980, Exo 1981, Ge´not 1990). The
area of nest chambers in piles of large stones
should also be bigger, and for many species of cav-
ity nesters, this is often correlated with larger
clutches and higher breeding success (e.g., Karls-
son and Nilsson 1977, Korpima¨ki 1985, Rendell
and Robertson 1989, Valkama and Korpima¨ki
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Figure 2. Cavity-entrance orientation of Little Owl nests
with and without success in the woodland area. Arrows
represent mean direction for each distribution and their
lengths correspond to the relative concentration (r; Zar
1996) of observations. Solid arrow—successful nests;
dashed arrow—failed nests.
1999). Finally, stone piles with larger stones are
usually more recent, less prone to erosion, and
thus, longer lasting ( Juillard et al. 1992).
Although predator frequency (predator occur-
rence in random points of both areas) seemed to
be similar between the two study areas, in the
steppe area Little Owls are apparently less able to
select nest sites without predators. The relative lack
of suitable cavities in this area possibly increases
the probability of occupation of the same stone
piles by both owls and predators. The selection of
cavities with more than one entrance (or exit)
seemed to have some importance in this area
(Glue and Scott 1980) and might be one strategy
to reduce the risk of predation.
Nesting Success. Tree diameter appeared to
be linked to the nesting success of Little Owls in
the woodland area, because successful nests were
located in trees with a greater DBH than failed
nests. By using an infrared micro-camera on 26
nests, we observed that trees with a larger diameter
seemed to hold deeper cavities, with more sinuous
and complex access tunnels than smaller diameter
trees. By nesting in trees with a larger girth, owls
possibly reduce the probability of a nest being
found by predators and may increase the ability of
adults and offspring to hide or to escape once
predators have found the nest. Other studies have
also demonstrated an inverse relationship between
depth of nest cavity and losses due to predation
(Moed and Dawson 1979).
Distance to human habitations appeared to be
associated positively with nesting success in the
study area, but as this variable was positively asso-
ciated with tree diameter, this may not represent a
cause-and-effect influence. Alternatively, the possi-
bility that predators were more numerous near hu-
man habitations could account for the higher pro-
portion of failed nests in those areas. However, no
observations indicating obvious differences in
predator density are available for our study area.
Entrances in the majority of failed nests in the
woodland area were towards north or northeast,
which may indicate that entrance orientation could
have influenced nesting success. Prevailing winds
and exposure to the sun may affect energy expen-
diture of adults and nestlings in some cavity-nest-
ing species, and thus influence cavity entrance ori-
entation (e.g., Lawrence 1967, Ricklefs and
Hainsworth 1968, Inouye et al. 1981, Valkama and
Korpima¨ki 1999). For many species of owls, cavity
orientation seems to be unimportant (Forsman et
al. 1984, Goad and Mannan 1987, McCallum and
Gehlbach 1988, Belthoff and Ritchison 1990),
while other Little Owl studies show that nest en-
trances may (Exo 1981, Ge´ not 1990) or may not
be ( Juillard 1980) protected against wind and rain.
Other variables related to nest sites might have
influenced nesting success in both study areas and
remained undetected. Some nests that failed very
early in the breeding season may not have been
detected resulting in some bias in our sample of
failed nests. Although this could have hampered
the identification of variables affecting nesting suc-
cess, we believe that very few failed nests were
missed and that it did not constitute an important
bias.
Several studies have focused on the effects of
large-scale and landscape variables on population
dynamics of Little Owls (e.g., Van Nieuwenhuyse
and Bekaert 2001, Van Nieuwenhuyse and Leysen
2001, Van Nieuwenhuyse et al. 2001, Ferrus et al.
2002). Our results show that smaller-scale features
associated with nest sites may also be important
and should be considered in management of Little
Owl habitat. In particular, management guidelines
directed towards Little Owl conservation in habi-
tats such as the ones considered in this study could
involve the preservation of large-diameter trees
and stone piles made of larger stones, as well as
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the maintenance of several alternative suitable cav-
ities in the owl territories.
A
CKNOWLEDGMENTS
We thank all those who assisted in the field work, es-
pecially Ineˆs Moreira, Carlos David Santos, Maria Peixe
Dias, Teresa Martins, and Joana Cardoso; Joe¨ l Bety and
Jari Valkama for statistical advice on logistic regression;
Caˆmara Municipal de Castro Verde and Liga para a Pro-
tecc¸a˜o da Natureza for logistic support; Paulo Catry, Esa
Lehikoinen, and Jari Valkama for providing valuable ad-
vice and comments on earlier drafts of this manuscript.
We also thank J. Bednarz, Jean-Claude Ge´not, Dries Van
Nieuwenhuyse, and an anonymous referee for important
suggestions on the final manuscript. R.Tome´ was funded
by Fundac¸a˜o para a Cieˆncia e Tecnologia (Programa
PRAXIS XXI, BD/5036/95).
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The Little Owl is the most common owl in the western Palearctic, and is featuring severe population declines in Central Europe. The sections on Habitat, Distribution, Population, Movements, Food, Social pattern and behaviour, Voice, Breeding, and Geographical variation have been updated or completely rewritten, and new sections added on Population Density, Population Regulation, Extra-pair Copulations and Extra-pair Young, Spatial Autocorrelation, and Conservation. The results of several new studies have been included. Most of these have been undertaken in Western Europe, and include research into habitat selection, breeding ecology in relation to territory density, food, and population dynamics in nest-box populations. Many new insights are included that stem from two recent international symposia on the species. An assessment of conservation activities in different parts of the range and research priorities following these results are given.
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Current recommendations for snag management, which focus on characteristics of individual snags or stocking levels in forests managed for timber production, not only obscure the importance of forest characteristics surrounding a potential nesting site but also fail to meet the management objectives or abilities of the small landowner. During the summers of 1978 and 1979, a random sample of 816 snags (standing dead trees) was studied in central New York. Sixty-seven percent of the sample was in mature maple (Acer spp.)-ash (Fraxinus spp.)-elm (Ulmus spp.), the remainder was in 2nd growth woodland. A set of 21 characteristics of snags and 19 characteristics of forest sites was analyzed to determine which best predicted bird use. Use was defined as the presence of ≥1 bird cavity in a snag or a forest site. Forest characteristics were sampled in randomly located 0.049-ha circles (N = 61) in 1979. Stepwise logistic regression revealed that forest characteristics (total snag basal area, tree species diversity, and number of tree species) were more reliable predictors of bird use than were snag characteristics (diameter at breast height [dbh], amount of bark, height, and species). Snag management based on selecting suitable forest sites and on maintaining or creating suitable snags within those sites is recommended. Use of logistic regression models by field managers is discussed.