Davor ĆIKOVIĆ*, Sanja BARIŠIĆ, Vesna TUTIŠ, Jelena KRALJ

Institute of Ornithology, Croatian Academy of Sciences and Arts, Gundulićeva 24, 10000 Zagreb, Croatia;

*e-mail: cikovicd@hazu.hr (corresponding author)

NEST SITE AND NEST-HOLE CHARACTERISTICS USED BY GREAT SPOTTED WOODPECKER DENDROCOPOS MAJOR L.

IN CROATIA

ABSTRACT: Great Spotted Woodpecker is the most abundant and widespread European woodpecker species, and it thus contributes the most to the number of excavated tree holes – an important habitat resource for secondary hole us-ers. However, majority of nest site characteristics data comes from boreal and temperate forests, with lack of information from Southern Europe. In this article, nest sites of the Great Spotted Woodpecker have been investigated in the conti-nental forests of Croatia – a previously understud-ied area of this species’ range. A total of 41 active nest-holes found in the breeding seasons 2003 and 2004 are described. Nest-holes were mainly positioned below the crowns, in injuries of branch abscission. Nesting tree species were not used randomly: wild cherry Prunus avium in hill and pedunculate oak Quercus robur in riverine forests were preferred while hornbeam Carpinus betulus and maples Acer sp. were avoided. While tree spe-cies used for nesting vary across the Great Spotted Woodpecker range, and thus cannot be used as a uniform nest site predictor, defected wood spots on a tree, like scars of branch abscission, are iden-tified as an important nest site clue and a habitat feature that is spatially more consistent. Nest-holes’ dimensions acquired in this research could not be clearly differentiated from those given for the other parts of the continent.

KEY WORDS: Great Spotted Woodpecker, Dendrocopos major, nest site, nest-hole character-istics

1. INTRODUCTION

Use of microhabitat features within a home range of an individual is a decision making process (Hutto 1985) that is as-sumed to be adaptive, suggesting that fitness is higher in preferred microhabitats (Mar-t in 1998). While indirect studies of habitat preferences (like correlations of population density to habitat type) often miss to identify selected microhabitat, measurement of habi-tat features that are directly chosen for certain activities can help identifying prefered habi-tat traits that affect fitness (Mart in 1998).

Almost all woodpecker species (Picidae) are excavators – primary hole-nesters (Short 1979). On a microhabitat scale selection they choose where to excavate a nest-hole (John-son 1980, Block and Brennan 1993, Jones 2001). The nest-hole protects offspring from predation and ensures optimal microclimate for its development (Short 1979, Ni lsson 1986, L i and Mart in 1991) thus a good choice of nesting site enhances fitness. More-over, nest-holes excavated by woodpeckers

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are an important habitat resource for many other species (e.g. Johnsson et al. 1993, Mart in and Eadie 1999, Kotaka and Mat-suoka 2002), and so woodpeckers are often considered ‘keystone species’ (e.g. Newton 1994, Wübbenhorst and Südbeck 2002, B ednarz et al. 2004).

The Great Spotted Woodpecker Dendro-copos major L. is the most abundant, the most widespread and the biggest habitat generalist among European woodpeckers (S cherzing-er 2001, Michalek and Miett inen 2003, Ćiković et al. 2008). Due to its abundance and distribution, the Great Spotted Wood-pecker contributes most to the number of excavated tree holes in a wide variety of ar-boreal habitats. Even though numerous stud-ies of the Great Spotted Woodpecker’s nest sites have already been conducted in Europe (e.g. Wesołowski and Tomiałojć 1986, Håg var et al. 1990, Smith 1997, Mazga-jsk i 1998, Kosiński and Winiecki 2004,

Kosiński and Ksit 2006, Pasinel l i 2007, Hebda 2009, Volke et al. 2010), the de-scription of nest-holes and nest sites from Southern Europe are still lacking. Ecological relationships between animals and their en-vironments are known to vary spatially and temporally so accounting for spatial hetero-geneity, when evaluating habitat selection processes of animals, is important for suc-cesful managment of a species (S outhwood 1977, Mcnew et al. 2013).

Characteristics of nest sites and nest-holes of the Great Spotted Woodpecker in Croatia are presented here for the first time. Nest site features were investigated on nesting trees and in a nearby habitat (12.5 m radius around the nesting tree). A comparison of the used to the available habitat resources was conducted in order to obtain indications of selectivity. Due to the regional differences in forest com-position and structure across Europe, differ-ences in the used nesting microhabitat were

Fig. 1. Map of the study area. 1. Riverine forest 120–140 years (300ha); 2. Hill forest 130 years (55 ha); 3. Hill forest 80–90 years (800 ha).

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expected when compared to the studies car-ried out in other parts of the continent. Nest-holes were measured and, as the Great Spot-ted Woodpeckers’ shows north-south clinal variation in size, differences in the nest-hole size were expected when compared to the northern populations. Mean regional popu-lation densities of the Great Spotted Wood-pecker in the study area were estimated to 5 pairs km-2 in mountain forests and to 11 pairs km-2 in lowland forests (Ćiković 2001).

2. MATERIALS AND METHODS

2.1. Study area

The research was carried out in the con-tinental forests of NW Croatia (45°N, 16°E). This area of diverse landscape of riverine low-lands, hills and mountains in the contact zone between Alps, Dinaric Alps and Pannonian Plain belongs to the temperate continental climate zone. The research was conducted at three sites: one was situated in lowland riv-erine forest and two were in hills (Fig. 1). All three sites were settled in large forest com-plexes. Total study area was 1155 ha.

Site 1 (45°35′N, 15°43′E) was in the 120–140 years old lowland riverine forest and it covered 300 ha. Narrow-leafed ash (Fraxinus angustifolia), black alder (Alnus glutinosa) and pedunculate oak (Quercus robur) domi-nated the forest stand occupying 89% of the total basal area of all trees, while black horn-beam (Carpinus betulus), field elm (Ulmus minor) and small-leaved lime (Tilia cordata) comprised 10% of the basal area.

Site 2 (45°54′N, 15°57′E) covered 55 ha and was situated at elevations between 400 and 500 m a.s.l. in a 130 years old forest stand. Common beech (Fagus sylvatica) and sessile oak (Quercus petraea) were the dominant tree species, occupying 95% of the total basal area. Other more abundant tree species were black hornbeam and sycamore maple (Acer pseudo-platanus).

Site 3 (46°10′N, 16°32′E) covered 800 ha and was situated in 80–90 years old forests on southward-exposed mountain flanks at eleva-tions between 250 and 440 m a.s.l. Common beech, sessile oak and black hornbeam domi-nated the forest stand with 88.5% of the total basal area. Sycamore maple, turkey oak (Quer-

cus cerris) and sweet chestnut (Castanea sati-va) comprised 6% of the total stand basal area.

2.2. Nest searches

The search for active nests was carried out in the second half of May and early June (earliest 13th May, latest 4th June) in 2003 and 2004 on all sites. While searching for nests field researchers relied on calls of nestlings since that is an efficient method of finding active nests (Mazgajski 1998, Kosiński et al. 2004, Volke et al. 2010). Some nests were omitted in this way; however, woodpeckers have high breeding success (Mart in and Li 1992, Mazgajski 2002b, Kosiński and Ksit 2006), and so the employed nest sam-pling method should not exhibit a strong bias due to focusing on successful nesting only. Each year, the search for nests lasted between 4 days (Site 2) and 10 days (Sites 1 and 3). Study sites were surveyed on foot in a grid pattern (series of parallel paths spaced 100 m apart). Between 1 and 3 people were involved in the search on each day, with average day/person output of about 25 ha covered on eas-ier grounds and 15 ha on difficult ones (steep slopes and/or dense vegetation). Measure-ments of nest-holes and habitat features were undertaken during September and October in order to avoid disturbing the nesting birds.

2.3. Nest-hole measurement

Nest-holes that were higher than 2 m were reached by climbing: nest-holes up to 10 m high were reached using tree climbing spikes, while those that were higher were reached using the single rope technique. Some of the nest-holes were unavailable due to poor condition of nesting trees. The following nest-hole features were recorded: nest-hole position on a nesting tree (in five categories: tree trunk, 1st level branch/bough, 2nd level branch, 3rd, or ≥4th branch); nest-hole placement in relation to different defects on a tree such as scars, wounds and similar features (in four categories: in a cir-cular scar of branch abscission, in a linear scar e.g., from lightning, in a proximity of such defects and distant from any kind of externally visible wood defects); condition of the nest-hole substrate was estimated vi-

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sually to one of the three categories (healthy, half-rotten and rotten). The following pa-rameters were measured: nest-hole depth and diameter, thickness of the front wall (± 1 cm), height and width of the entrance (± 1 mm), diameter of the trunk/branch on the nest entrance position (± 1 cm), incli-nation and direction of the entrance (± 1 degree) and height above the ground (to the entrance ± 0.1 m). The interior chamber parameters were measured by inserting a light bulb on a wire into the nest-hole, after which the chamber depth was measured us-ing a plumb, the width was measured with a ruler and the chamber’s interior was in-spected using a dental mirror.

2.4. Nesting trees description

The nesting tree species was identified and the following parameters were measured: diameter at breast height (further on abbrevi-ated as DBH – diameter of a tree at the height of 1.5 m approximately; measured with tape measure to ± 1 cm), tree height (with clinom-eter to ± 1 m), height of the trunk (from the forest floor to the first branch of a distinctive crown, with tape measure to ± 0.1 m).

Condition of the nesting tree was vi-sually classified in one of three categories: alive/healthy (external structural defects or mechanical damage absent or insignificant, crown appears healthy), fairly vital (has mi-nor structural defects or mechanical dam-age, some disease signs and crown thinned, but no obvious signs of decay and appears reasonably vital) and poor condition/dead (apparently unhealthy trees with many or serious structural defects and dead parts like fissures of the bark, trunk/branches decay, thin or fallen crown and present sporocarps). Furthermore, we visually estimated the pro-portion of dead branches in the crown to the following categories: healthy crown (<40% of dead branches), half-dried crown (40–60% dried branches) and dead crown (>60% dried branches, including ‘stems’ i.e. trees without crowns). Old nest-holes, if observed any-where on the nesting tree, were counted (in four categories: 0, 1–2, 3–5 and >5) and pres-ence of woodpeckers’ feeding marks were noted (damages on the tree surface created by feeding woodpeckers, Y/N data).

2.5. Habitat description

Habitat features at nest sites were sam-pled in circular plots of a 12.5 m radius (0.05 ha) around each nesting tree (James and Shugar t 1970, Cyr and Oelke 1976, B onnot et al. 2009). On each plot (further on: Nest Plot, abbreviated as ‘NP’ i.e. ‘NPs’ for plural), all tree species were identified and each tree DBH was measured. This enabled the description of each species of tree on NP in terms of the Number of Trees (per hectare) and the Basal Area (m2 ha-1). Average density and height of shrubs was visually estimated for the whole NP (± 10% density and ± 0.5 m height). Shrubs were defined as wooden plants in form of bush and young trees less than 3 m high and less than 5 cm DBH.

Habitat data of the whole study sites, which were compared to habitat data of NPs, were taken from forestry management plans. The Number of Trees (per hectare) and the Basal Area (per hectare) were used for each species of tree in all study sites. Dendrometric sampling methods used for management plans (Pranj ić and Lukić 1997) were consistent with sam-pling methods used on NP, so the habitat data was used without transformations.

2.6. Data analysis

Due to the differences in habitat types be-tween study sites, habitat related parameters were analyzed separately for lowland forests (Site 1) and hill forests (Sites 2 and 3). Prior to pooling, habitat data for Sites 2 and 3 was tested for the differences in the Basal Area (t = 0.29, d.f. = 8, P = 0.78) and the Number of Trees (t = 0.95, d.f. = 8, P = 0.36). Param-eters related to nest-hole size, direction, incli-nation and position on a tree were analyzed jointly for all study sites (testing for differ-ences between lowland and hills showed no significant differences).

All continuous variables of nest-hole characteristics were normally distributed (Shapiro-Wilk W test: W = 0.920–0.984, P = 0.052–0.946) with the exception of nest-hole entrance width, which was log-trans-formed resulting in normality (W = 0.974, P = 0.763).

To test the Great Spotted Woodpecker’s selectivity for nesting tree diameter, DBH of

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nesting trees per tree species were compared to the average DBH of NPs per tree species using the t-test. Trees with DBH <21 cm were excluded (the thinnest nesting tree used in this study), since they were considered not adequate for nesting.

To test whether the slope exposition and/or slope inclination influenced the nest-hole entrance directions in hills, these parameters were correlated to the nest-hole entrance di-rection using circular-circular correlation by Fisher and Lee (1983). All nest-hole en-trance directions were tested for preference using the Rayleigh uniformity test (Fisher 1993). In addition, nest-holes in lowland for-ests were separately tested for direction pref-erence because these could not be influenced with the exposition of the slope.

To check the selectivity of the nesting tree species, Manly’s standardized selection index for sampled resources was used (Manly et al. 1993). The number of trees of each tree spe-cies on all NPs (available resource) was com-pared to the number of used nesting trees (used resource). Only trees with DBH ≥21 cm were considered as available resources. Con-fidence limits (corrected with Bonferroni correction for α/n) were computed for the selection indices and, if selection confidence limits of tree species pair did not overlapped, the statistical significance of certain tree spe-cies selectivity was checked.

To compare the habitat of NPs to Study Sites, average DBHs of trees (per species of tree) on NPs and Study Sites were tested for differences using the t-test. In addition, the level of similarity of average DBHs between NPs and Study Sites was expressed with sim-plified Morista’s index of similarity (Horn 1966, Krebs 1999). This index is nearly inde-pendent of the sample size and Wolda (1981) recommended it as the best overall measure of

similarity for ecological use. It varies from 0 (no similarity) to 1 (complete similarity).

Descriptive statistics and basic tests were calculated in Statistica 8 (StatSoft Inc. 2007). All angular statistics was calculated in the Oriana software (version 3.21., Kovach Com-puting Services 1994–2010). Morista’s simi-larity and Manly’s selection tests were cal-culated in Ecological Methodology ver. 6.1 (Kenney and Krebs 2003).

3. RESULTS

In two consecutive breeding seasons, 2003 and 2004, 41 active nest-holes were found in total: 19 nest-holes on Site 1, five on Site 2 and 17 on Site 3. Based on the estimated population densities, approximately one third of expected active nest-holes were found on the study sites.

3.1. Nesting site

Most nest-holes were excavated in tree trunks (78%, Fig. 2). On average, nest-holes were positioned in the upper half of trunk: tree crowns took about half of tree height, while nest holes were at about the first third of tree height (Table 1). There was signifi-cant positive modest correlation between the relative trunk height and the nest position (r = 0.4, P = 0.03, n = 27).

Furthermore, about two thirds of nest-holes (67.5%, n = 40) were positioned in exter-nally visible wood defects: in scars of branch abscission (27.5%), in linear scars (15%) and in proximity of such wounds (25%). The re-maining nest-holes (32.5%) were not placed in any kind of externally visible wood defect.

Nesting tree species were not used ran-domly (χ2

LOWLAND = 15.12, d.f. = 5, P = 0.01; χ2

HILLS = 29.23, d.f. = 8, P = 0.0003). In lowland

Table 1. Average height of nesting tree, its major parts and nest-hole; relative heights of crown and nest-hole to nesting tree. *The highest nest hole was estimated at 20 m but could not be reached for exact measurement.

n Mean S.D. RangeTree (m) 38 26.30 6.69 11–41Trunk (m) 29 12.90 5.44 3–26Crown (m) 27 13.70 8.17 2–34Hole (m) 28 7.75 3.76 1.80–16*Crown / tree 27 0.49 0.23 0.10–0.90Hole / tree 28 0.31 0.17 0.06–0.72

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riverine forest (Site 1), six tree species were available for nesting, but four of them were used: ash, alder, oak and elm. Majority of nests were in alder and ash, the most numer-ous tree species of the stand, and they were used in accordance with their availability. In contrast, pedunculate oak was used more

than expected from random, indicating pref-erence, although the latter just approached statistical significance (χ2 = 3.32, P = 0.07; Table 2). In hills (Sites 2 and 3), nine tree species were available, but four were used for nesting: sessile oak, beech, wild cherry (Prunus avium) and chestnut. A similar num-

Fig. 2. Position of the Great Spotted Woodpecker nests on nesting trees (n = 41).

Table 2. Selection indices for the nesting tree species. Selection indices (w) above 1.0 indicate preference and values less than 1.0 indicate avoidance (Manly et al. 1993). Standardised selection indices (B) are standardised ratios that sum to 1.0 for all resources where ratios of (1/Noresources) indicate no prefer-ence, values below this indicate relative avoidance and values above indicate relative preference (Manly et al. 1993).Riverine forest

Tree sp. Count Avail-able

Proportion Available Count Used Proportion

Used Sel. Index (w) Stand. Sel. Index (B)

Fraxinus angustifolia Vahl 85 36.6% 6 31.6% 0.86 0.14

Alnus glutinosa L. 48 20.7% 7 36.8% 1.78 0.29Ulmus minor Mill. 38 16.4% 1 5.3% 0.32 0.05Carpinus betulus L. 30 12.9% 0 0.0% 0.00 0.00Quercus robur L. 19 8.2% 5 26.3% 3.21 0.52Acer campestre L. 12 5.2% 0 0.0% 0.00 0.00

Hill forests

Tree sp. Count Avail-able

Proportion Available Count Used Proportion

Used Sel. Index (w) Stand. Sel. Index (B)

Quercus petraea Liebl. 116 41.4% 8 36.3% 0.87 0.06Fagus sylvatica L. 104 37.1% 8 36.3% 0.98 0.07Carpinus betulus L. 28 10.0% 0 0.0% 0.00 0.00Acer pseudoplatanus L. 15 5.4% 0 0.0% 0.00 0.00Prunus avium L. 6 2.1% 5 22.7% 10.6 0.71Castanea sativa Mill. 5 1.8% 1 4.5% 2.5 0.17Acer campestre L. 3 1.1% 0 0.0% 0.00 0.00Acer platanoides L. 2 0.7% 0 0.0% 0.00 0.00Quercus cerris L. 1 0.4% 0 0.0% 0.00 0.00

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bers of nests were found in oak and beech, which were used in accordance to availability, whereas cherry was used significantly more than expected from random (χ2 = 15.07, P <0.001; Table 2). Hornbeam, as well as dif-ferent species of maples that were present on all study sites, were not used despite their fairly high availability.

Nesting tree conditions as well as pro-portions of dead branches in the crowns of nesting tree, were evenly distributed. Nesting trees were 39% alive or mostly alive, 24% were fairly vital, and 37% were in poor condition or dead; moreover, crowns were 43% healthy, 19% half-dried, and 37% dried (n = 39).

Most nests were excavated in trees of 30–50 cm DBH (69 %), with average diam-eter in range of 45 ± 5 cm (n = 38, Table 3). The thinnest nesting tree was a chestnut tree of 21 cm DBH. The average DBH of the nest-ing tree (45.26 cm, Table 4) did not differ from the average DBH of the NPs (39.44 cm; t = 1.33, n = 38, P = 0.2).

Most nesting trees had additional wood-pecker nest-holes (86% of nesting trees, n = 34), with most having one or two of them (59%). Feeding marks have not been found on most nesting trees (76%, n = 41).

Average shrub height on the NPs was 3.3 m (range = 0.5–5.0 m) and shrub cover-age was 34% (range = 5% – 100%; n = 29, NPs without shrubs were excluded). Nest heights were not influenced by the presence (r = –0.1, P = 0.55, n = 37) and height (r = –0.02, P = 0.92, n = 19) of the shrubs.

No difference was found in forest struc-ture between the NPs and the Study Sites: comparison of average DBHs for 20 tree species/cases in total showed no significant difference (t = –0.53, d.f. = 38, P = 0.6), and Morista’s index of similarity was high for all plots (CHSITE–1 = 0.98, CHSITE–2 = 0.97, CHSITE–3 = 0.90).

3.2. Nest-hole dimensions

We could reach 28 nest-holes (68% of the total number recorded). Their dimensions and descriptive statistics are presented in Table 4. Entrances could be described as more circu-lar than oval: with tolerance of 2 mm width-height difference, 52% of entrances were circu-lar, whereas we found no prevalence in width or height stretch in remaining entrances. Al-most all nest entrances were inclined down-wards (96%). The entrance direction showed ESE mean, but directions were scattered and showed no statistical preference (Rayleigh Z = 0.5, P = 0.6, n = 41, Fig. 3). In addition, number of entrances facing ± 90° around mean direction was compared to opposite facing di-rections and the difference was insignificant (χ2 = 0.59, P = 0.44, n = 41). The lack of pre-ferred direction was also noted across the sample of the nests in the lowlands only (Ray-leigh Z = 0.32, P = 0.73, n = 22). In hills, the entrance direction was not influenced by the slope exposition or slope inclination (r = 0.047 and r = 0.048, n = 19). Thickness of the front wall was not correlated with the diameter of

Table 3. Comparison of average tree diameters at breast height (DBH) between nesting trees (NT), trees of nest plots (NP – trees thicker than 20 cm) and trees of the Study Sites (Site).

Average DBHTree species NT NP Site

Site 1

Alnus glutinosa L. 40.0 34.8 32.4Fraxinus angustifolia Vahl 51.0 46.3 46.4

Quercus robur L. 55.0 52.1 70.8Ulmus minor Mill. 35.0 33.4 26.0

Site 2+3

Fagus sylvatica L. 46.4 34.6 30.5Quercus petraea Liebl. 45.0 40.1 34.5

Prunus avium L. 43.0 35.0 29.0Castanea sativa Mill. 25.0 35.0 26.7

Mean 42.5 38.91 37.04S. D. 9.4 6.82 15.07

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the trunk/branch at nest height (r = -0.05, n = 25, P = 0.796). Substrate in which holes were excavated was estimated as rotten in 80% and healthy in 17% of the cases (n = 35).

4. DISCUSSION

In order to distract competitors and pred-ators, nest-hole entrances are excavated in the smallest possible dimensions (Short 1979, Hebda 2009). Out of two widely distribut-ed Great Spotted Woodpecker subspecies in Europe, D. m. major (northern and eastern Europe) and D. m. pinetorum (north-west-ern and south-eastern Europe), the latter is slightly smaller (Michalek and Miett inen 2003) and it inhabits Croatia (Winkler and

Christ ie 2002). Therefore, we expected to find smaller nest-hole entrances compared to those in northern populations. This assump-tion was supported by an average entrance size in Estonia (20.81 cm2, n = 21, Volke et al. 2010), which was bigger than the aver-age presented here (17.72 cm2, n = 26). Data from Poland (17.77 cm2, n = 63, Kosiński and Ksit 2007) is very similar to ours but it comes from a subspecies border area. Aver-age entrance size from Sweden was the small-est (17.32 cm2, n = 8, Carlson et al. 1998) and it did not support our assumption, but this could be attributed to a small sample size.

Our research showed no preference in nest-hole entrance direction. Similar lack of preference, ESE tendency and lack of overall E to W directions were presented by Volke et al. (2010) in Saarema, Estonia. Other anal-yses of Great Spotted Woodpecker entrance direction in general showed no clear prefer-ence or uniform direction (Wesołowski and Tomiałojć 1986, Mazgajski 1998, Mi-chalek and Miett inen 2003, Kosiński and Kempa 2007, Hebda 2009 etc.). In contrast to direction, entrances were almost uniformly inclined downwards. The same was found in some other woodpecker spe-cies (C onner 1975, Hooge et al. 1999) but wasn’t yet featured for Great Spotted Wood-pecker. It is likely that woodpeckers deliber-ately select entrances in downward inclined places to avoid precipitation entering the nest, as suggested by C onner (1975).

Even though the Great Spotted Wood-pecker is the most versatile European wood-

Table 4. Dimensions of nest-holes. Entrance inclination values are presented as departures from the vertical: negative values are upward inclined and positive are downward inclined departures from the vertical.

n Mean S.D. Range

H-entrance 25 4.82(Mode = 4.6) 0.39 4.3–5.6

W-entrance 25 4.70(Mode = 4.6) 0.47 3.7–5.8

Width – Height 25 –0.14 0.44 –1.0–0.8Front wall 25 5.56 1.83 3.0–9.2Chamber depth 26 22.10 7.43 12.5–42.0Chamber width 26 17.17 3.39 11.0–25.0Diameter at nest height 26 36.35 11.44 17.0–61.4Entrance inclination 25 11.56° 10.65° (–7.0°)–(42.0°)Entrance orientation 41 108.3° 120.10° 7.0°–358.0°

 

N

E

S

W

6

6

5

5

4

4

3

3

2

2

1

1

Fig. 3. Orientation of nest entrances (n = 41). Dis-tribution histogram, mean (108.2°, bold black ra-dial line) and standard deviation (± 120.1°. bold grey outline).

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pecker, we found certain patterns in habitat parameters used for nesting: trees used for nesting were not a random set of available trees and nest positions on trees can be de-scribed by a relatively narrow set of attri-butes. Irrespectively of the nesting tree spe-cies or condition, nest-hole positions could be described using the following attributes:• Nest-holes were excavated mainly in live

trees, in isolated injuries created mostly in places of branch abscissions and in substrate that was mostly estimated to be rotten.

• Nest-holes were mainly located in the upper half of trunks. High proportion of nest-holes in trunks may be conditioned with minimal requirements for nest size, but also with the fact that trunks are the oldest parts of trees with highest abun-dance of scars originating from branch abscission. Nest-hole position was not af-fected with the presence of shrubs.

• High proportion of used trees already had old nest-holes (86%), in most cases one or two of them. In contrast to our findings, some authors

(e.g. Håg var et al. 1990, Mazgajski 1998, 2002a) found that most nest-holes were ex-cavated in trees without old holes (61–71%). Such difference could be an outcome of dif-ferent methodologies. For instance, Mazga-jsk i (1998, 2002a) noted only old nest-holes nearby the active one, while we noted nest-holes without distance limitations. As nest predation is the main cause of mortality in the nesting period (Ni lsson 1986, Mar-t in 1995) and Pine Marten Martes martes searches for larger woodpecker holes and even revisits them from year to year (S on-erud 1985), selection of trees without old nest-holes was explained as an anti-predator behavior (Håg var et al. 1990, Mazgajski 2002a). However, selection of trees with old nests-holes may have some advantages in de-creasing predation too: in order to prevent loss of the clutch adults may choose places with more opportunities for nest sites so that predators need to invest more time in search-ing (Li and Mart in 1991, Mart in and Roper 1988).

In most cases, the most available tree species was used for nesting; however, we found that tree species were not used at ran-

dom. Compared to availability, there was a non-proportionally high use of wild cherry in hills and pedunculate oak in riverine for-ests. Many authors already emphasized the use of oaks (Jenni 1983, Spitznagel 1990, Kosiński and Winiecki 2004, Pasinel l i 2007 etc.) while cherry was rarely mentioned (Michalek and Miett inen 2003) probably because it is scattered and rare throughout its range (Russel l 2003). Other frequently used tree species mentioned in this study were also often used in different parts of the Great Spotted Woodpeckers’ range (Glutz von Blotzheim and Bauer 1980, Mi-chalek and Miett inen 2003). Even though it was found to be used in some parts of the range (Wesołowski and Tomiałojć 1986, Mazgajski 1998), we found that hornbeam has been avoided. Together with different maple and elm species, hornbeam formed an understory that appears to be avoided in our study areas with partiality for trees that formed the main canopy.

Most nest-holes were excavated in hard wood trees (Kretschmann 2010) so it may seem that wood hardness, even though hard wood is more difficult to chisel, was not a crucial factor affecting the nest site. How-ever, birds frequently used spots with dead substrate and thus avoided excavating in hard woods. For example, even though cherry is classified as a hard wood species it is more prone to wood defects (like heart rot, frost crack, heart shake etc.) than other broad-leaved forest tree species (Thi l l 1980); thus it is a rich source of soft substrate, which could explain its use for nesting. Furthermore, all nests in beech (7 nests) and almost all in oaks (10 of 12) were in scars – that is, in wood of disrupted structure. Excavation of nest-holes in soft substrate seems to have been favored over sound wood (Jackson and Jackson 2004), and that corresponds to a preference of soft wood species over the hard ones in other parts of species range (Rolstad et al. 1995, Mazgajski 1997, 1998, Weiss 2003). Thus, availability of soft substrate in a firm wood, rather than tree species’ specific hardness, seems to be a decisive clue that affects nest-ing site choice. It seems that the Great Spotted Woodpecker selects trees with specific physi-cal characteristics, irrespective of tree spe-cies, and is able to identify variations in wood

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hardness among trees and among positions within a tree (Matsuoka 2008).

Habitat selection is a spatially non-uni-form process and species use different habi-tats across their range; thus, availability of the data on habitat use from different areas is considered important for sound bird-habitat modeling and management. However, we suggest that, apart from inconsistent large scale habitat features (e.g. forest composi-tion and structure), there is a set of smaller scale habitat features (e.g. isolated injuries of trees) that the Great Spotted Woodpecker uses much more consistently across its range and recognition of such features could facili-tate understanding of bird-habitat relation-ship. According to this research, availability of isolated wood defects may be the main clue affecting nest site choice, while tree species might serve as an indicator of availability of such properties.

ACKNOWLEDGMENTS: We thank Ivan Budinski and Ognjen Vukadinović for help in the search for the nests, Public institution ‘Nature Park Medvednica’ who granted us permission to conduct research in the Nature Park and to For-est Administration Subsidiaries ‘Jastrebarsko’, ‘Zagreb’ and ‘Križevci’ for providing us with large-scale habitat data.

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