Polish Journal of Ecology Pol. J. Ecol. (2014) 62: 349–360
Regular research paper
Davor ĆIKOVIĆ*, Sanja BARIŠIĆ, Vesna TUTIŠ, Jelena KRALJ Institute of Ornithology, Croatian Academy of Sciences and Arts, Gundulićeva 24, 10000 Zagreb, Croatia; *e-mail:
[email protected] (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 users. 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 continental forests of Croatia – a previously understudied 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 species 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 identified as an important nest site clue and a habitat feature that is spatially more consistent. Nestholes’ 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 characteristics 1. INTRODUCTION Use of microhabitat features within a home range of an individual is a decision making process (Hutto 1985) that is assumed to be adaptive, suggesting that fitness is higher in preferred microhabitats (Mart i n 1998). While indirect studies of habitat preferences (like correlations of population density to habitat type) often miss to identify selected microhabitat, measurement of habitat features that are directly chosen for certain activities can help identifying prefered habitat traits that affect fitness (Mar t i n 1998). Almost all woodpecker species (Picidae) are excavators – primary hole-nesters (Shor t 1979). On a microhabitat scale selection they choose where to excavate a nest-hole (Joh ns on 1980, Bl o ck and Bre n nan 1993, Jone s 2001). The nest-hole protects offspring from predation and ensures optimal microclimate for its development (Shor t 1979, Ni lss on 1986, L i and Mar t i n 1991) thus a good choice of nesting site enhances fitness. Moreover, nest-holes excavated by woodpeckers
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are an important habitat resource for many other species (e.g. Johnss on et al. 1993, Mar t i n and E adie 1999, Kot a k a and Mat suoka 2002), and so woodpeckers are often considered ‘keystone species’ (e.g. Ne w ton 1994, Wübb en horst and Südb e ck 2002, B e d narz et al. 2004). The Great Spotted Woodpecker Dendrocopos major L. is the most abundant, the most widespread and the biggest habitat generalist among European woodpeckers (S cherz i ng er 2001, Micha lek and Miett inen 2003, Ć i kov i ć et al. 2008). Due to its abundance and distribution, the Great Spotted Woodpecker contributes most to the number of excavated tree holes in a wide variety of arboreal habitats. Even though numerous studies of the Great Spotted Woodpecker’s nest sites have already been conducted in Europe (e.g. We s o ł owsk i and Tomi ał oj ć 1986, Håg var et al. 1990, Smit h 1997, Ma z g a jsk i 1998, Kosińsk i and Winie ck i 2004,
Ko s i ńsk i and K s it 2006, Pas i nel l i 2007, Heb d a 2009, Vol ke et al. 2010), the description of nest-holes and nest sites from Southern Europe are still lacking. Ecological relationships between animals and their environments are known to vary spatially and temporally so accounting for spatial heterogeneity, when evaluating habitat selection processes of animals, is important for succesful managment of a species (S out hwo o d 1977, Mc ne w 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 composition and structure across Europe, differences 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).
Great Spotted Woodpecker nest-holes and nest sites in Croatia
expected when compared to the studies carried out in other parts of the continent. Nestholes were measured and, as the Great Spotted Woodpeckers’ shows north-south clinal variation in size, differences in the nest-hole size were expected when compared to the northern populations. Mean regional population densities of the Great Spotted Woodpecker in the study area were estimated to 5 pairs km-2 in mountain forests and to 11 pairs km-2 in lowland forests (Ći kov i ć 2001). 2. MATERIALS AND METHODS 2.1. Study area The research was carried out in the continental forests of NW Croatia (45°N, 16°E). This area of diverse landscape of riverine lowlands, 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 riverine forest and two were in hills (Fig. 1). All three sites were settled in large forest complexes. 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) dominated the forest stand occupying 89% of the total basal area of all trees, while black hornbeam (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 pseudoplatanus). 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 elevations between 250 and 440 m a.s.l. Common beech, sessile oak and black hornbeam dominated the forest stand with 88.5% of the total basal area. Sycamore maple, turkey oak (Quer-
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cus cerris) and sweet chestnut (Castanea sativa) 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 (Ma z g aj sk i 1998, Ko s ińsk i et al. 2004, Vol ke et al. 2010). Some nests were omitted in this way; however, woodpeckers have high breeding success (Mar t i n and L i 1992, Ma z gaj sk i 2002b, Ko s i ńsk i and K s it 2006), and so the employed nest sampling 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 easier grounds and 15 ha on difficult ones (steep slopes and/or dense vegetation). Measurements 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 circular 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 parameters 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), inclination 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 using a plumb, the width was measured with a ruler and the chamber’s interior was inspected 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 abbreviated as DBH – diameter of a tree at the height of 1.5 m approximately; measured with tape measure to ± 1 cm), tree height (with clinometer 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 visually classified in one of three categories: alive/healthy (external structural defects or mechanical damage absent or insignificant, crown appears healthy), fairly vital (has minor structural defects or mechanical damage, 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 proportion of dead branches in the crown to the following categories: healthy crown (60% dried branches, including ‘stems’ i.e. trees without crowns). Old nest-holes, if observed anywhere on the nesting tree, were counted (in four categories: 0, 1–2, 3–5 and >5) and presence 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 sampled in circular plots of a 12.5 m radius (0.05 ha) around each nesting tree (James and Shugar t 1970, Cy r and O el ke 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 (Pranjić and Lu k ić 1997) were consistent with sampling methods used on NP, so the habitat data was used without transformations. 2.6. Data analysis Due to the differences in habitat types between 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). Parameters related to nest-hole size, direction, inclination and position on a tree were analyzed jointly for all study sites (testing for differences 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 nesthole entrance width, which was log-transformed resulting in normality (W = 0.974, P = 0.763). To test the Great Spotted Woodpecker’s selectivity for nesting tree diameter, DBH of
Great Spotted Woodpecker nest-holes and nest sites in Croatia
nesting trees per tree species were compared to the average DBH of NPs per tree species using the t-test. Trees with DBH
