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Article

Habitat Requirements of the Grey-Headed Woodpecker in Lowland Areas of NE Poland: Evidence from the Playback Experiment

by
Grzegorz Zawadzki
1,* and
Dorota Zawadzka
2
1
Department of Forest Protection, Institute of Forest Sciences, Warsaw University of Life Sciences, Nowoursynowska 159, 02-776 Warszawa, Poland
2
Department of Forest Sciences, Branch in Tomaszów Mazowiecki, University of Łódź, Konstytucji 3 Maja 65/67, 97-200 Tomaszów Mazowiecki, Poland
*
Author to whom correspondence should be addressed.
Birds 2025, 6(3), 32; https://doi.org/10.3390/birds6030032
Submission received: 23 March 2025 / Revised: 11 June 2025 / Accepted: 12 June 2025 / Published: 20 June 2025
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Simple Summary

The grey-headed woodpecker is one of the least-studied European woodpeckers. It is protected under Annex I of the Birds Directive. It inhabits mainly deciduous and mixed forests, but its detailed habitat preferences are poorly recognized. We assessed the abundance and habitat requirements of the species in the Augustów Forest (NE Poland), an important refuge included in the Natura 2000 network. The best explanatory predictors increasing the probability of woodpecker presence among the variables analyzed were watercourses, a high proportion of mixed forest area, and a proportion of stands older than 120 years. To effectively protect the habitats of the grey-headed woodpecker, it is necessary to maintain some areas of stands over 120 years old on wet sites.

Abstract

The grey-headed woodpecker (Picus canus) (GHW) is one of the least-studied European woodpeckers, listed in Annex I of the Birds Directive. We examined the key environmental characteristics that determine the possibility of GHW occurrence in vast forests in northeast Poland. Woodpeckers were inventoried in spring on 54 study plots (4 km2) covering 20% of the forest area. Active territories were detected and mapped using the playback experiment of territorial voices and drumming. The generalized linear model GLM, random forest RF, and Boosting were used for modeling. GLM was used to indicate the most critical factors affecting the abundance of GHW. The number of territories in a single study plot ranged from 0 to 3; the most frequent were areas without woodpeckers. The probability of the nesting of the GHW was increasing at plots with watercourses, a bigger share of mixed forest area, and a proportion of stands over 120 years old. The calculation for all 400 quadrats allowed us to estimate the population size at approximately 180–200 breeding pairs. The overall density of GHW in the study area was assessed at 0.13/km2, while at the optimal quadrats, it increased to about 0.75/km2. Preference for watercourses was linked to alders growing along water banks. Near the water, there are often small meadows where the GHW can prey on ants. In turn, old-growth forests above 120 years old increased the probability of the presence of the GHW. There are more dead and dying trees in older forests, which are the ones the GHW chooses to excavate cavities. To effectively protect the habitats of the GHW, it is necessary to maintain a larger area of stands over 120 years old, mainly on wet sites.

1. Introduction

The grey-headed woodpecker (hereafter GHW) (Picus canus) is one of the least-studied European woodpeckers [1]. Most of the information has been published in studies on other woodpeckers [2]. Its range runs along the mid-latitudes, covering almost all of Europe, excluding the British Isles, the Iberian and Apennine Peninsulas, and continuing through the temperate parts of Asia up to Japan [3,4]. The species reaches high densities in central Europe’s mountain forests and in lowland forests in the Baltic States [4]. The European population was estimated at 365,000–609,000 mature individuals, with 182,000–305,000 breeding pairs, and comprises approximately 40% of the species’ global range [5]. Despite some fluctuations, the overall trend in Europe is increasing, especially in the northern part of the area, e.g., in Scandinavia [4,6]. GHW is a sedentary bird, but may occur outside woodland areas as part of post-breeding dispersal, in addition to winter movements at the northern limit of its range [4]. The northward expansion of the species’ range is linked to a warming climate. Lower snow cover or a periodic lack of snow cover promotes a better availability of the woodpecker’s food [7,8].
In Poland, the distribution of the GHW is uneven. Its breeding area mainly covers the southern and northeastern parts of the country. There are isolated breeding sites between these areas, which are associated with the expansion of the range of the GHW [9]. The size of the Polish population was assessed at about 3000–5000 breeding pairs [10]. Still, no data exists on this species’ long-term population trends [11]. Nevertheless, this species’ range expansion in Poland has been documented [12,13].
The GHW is listed in Annex I of the EU Birds Directive [14]. Habitat preferences of the grey-headed woodpecker have not been fully recognized, which makes it difficult to effectively protect the habitat of this species following the Birds Directive. According to Kosiński et al. [9], this species selects mainly deciduous and mixed forests. The GHW is considered a deciduous forest specialist [15]. It prefers old and extensively managed forests, but its habitat preferences vary significantly across the European range. In some parts of Europe, GHW inhabits old-growth beech forests and mixed deciduous and coniferous stands, and in others, it is most common in floodplain willows and poplars. It often occurs in areas of mosaic forests and open areas with accessible feeding grounds [3]. In Norway, the GHW was abundant in forests with old aspen trees, more numerous at the edges of the stand. Spruce plantations did not impact woodpecker numbers [16]. In Poland, the GHW occurs in deciduous stands of various types with low crown density. In the south of the country, a large proportion of the population is associated with stands composed of beech trees (Fagus sylvatica) [9]. It also inhabits riverside riparian forests, alder forests, mixed forests, and large parks and woodlots. The GHW rarely occupies coniferous forests in the vicinity of deciduous stands. Home ranges of the GHW are often distributed near open areas [9]. The GHW frequently used a mosaic of habitats with patches of old-growth forest, young forest plantations, and clearings [17]. According to Rolstad and Rolstad [7], an essential habitat element is the presence of single, old trees. Moreover, dead or weak trees are needed to excavate cavities in them [18,19,20]. As a primary excavator, the GHW creates cavities that are used next for a long time by medium-sized and small birds for breeding and resting [21]. Except for the characteristics of GHW nest trees, there is scarce information on what habitat parameters are essential for this species. This study aimed to identify the key environmental factors that determine the possibility of GHW occurrence and assess population numbers in a large compact forest complex in northeastern Poland.
Firstly, we hypothesized that the occurrence of the GHW will depend on the presence of watercourses. We predicted that the occupancy of the GHW will increase with the availability of watercourses because of the spatial distribution of optimal breeding habitats formed by moist forests. However, lakes will not be a good predictor as there are only small tracts of deciduous moist forests along lake shores. Secondly, we hypothesized that GHW numbers would be related to two parameters: the presence of old stands of deciduous trees providing potential breeding sites and areas with low vegetation (meadows and clearings) used for foraging. Correspondingly, we predicted that a combination of higher shares of these two parameters favor higher densities of GHWs.

2. Materials and Methods

2.1. Study Area

The Polish part of the Augustów Forest is located in the country’s northeast (at 23°15′ E, 53°54′ N), extending over 1400 km2. The area is relatively flat, with elevations between 135 and 190 m a.s.l. The climate is rather cold, with a mean annual temperature of 6.5 °C [22]. The forest cover is approximately 90%, whereas lakes account for 6% of the area. Augustów Forest is located in the basin of four rivers. Also, there are 25 smaller rivers and streams with 15 canals across the study area. The tree stands were dominated by the Scots pine (78%), Norway spruce (Picea abies) (8%) growing almost on all forest site types, black alder (Alnus glutinosa) (9%) growing mainly on wet sites, silver birch (Betula verrucosa) (5%), and pedunculate oak (Quercus robur) (1%). A large part of the area is covered by single-generation forests, with trees of a similar age formed by one dominant tree species with admixtures of several others. The average age of tree stands is 65 years, and stands older than 100 years account for approximately 15% of the forest area [23]. Among the forest site types, mesic pine forests accounted for almost 40% of the area, while a further 27% came from mesic mixed forests. Approximately 7% of the forest is covered by wet forest sites. Augustów Forest is included within Europe’s Natura 2000 network as the Special Protection Area for Birds PLB200002. The “Puszcza Augustowska” woodpecker is an object of protection in this area. Most of the area comprises commercial stands managed by six forest districts of Poland’s State Forests National Forest Holdings. The Lake Wigry National Park covers an area of 150 km2 and is located in the northwestern part of the Augustów Forest [19,22].

2.2. Field Work

The study was conducted between 2018 and 2021 in the Augustów Forest. The area of the entire forest complex is covered by a grid of 400 2 × 2 km squares (following Sikora and Kosiński [17]). Subsequently, 54 randomly selected squares covering approximately 20% of the forest area, representative of Augustów Forest (in accordance with the bird monitoring methodology [17]), were drawn using ArcMap, and research was conducted. At the beginning of each breeding period in the study plots, woodpecker territories were detected and mapped using a playback experiment of territorial voices and drumming. Fieldworks were conducted during March, April, and the first half of May, between 7 and 9 am. During the breeding season, counts were carried out twice in each quadrat, 20–30 days apart, only in good, windless, and rain-free weather. In each square, 10–12 listening points were designated. Points were distributed along two transects, within 400 m of each other, on average. At each of them, a drumming and territorial voice was played for 2 min and listened to for 3 min [17].
All observations were mapped on-going during the fieldwork. Most birds responding were seen at a short distance (20–200 m) from the counting point. Birds with voices coming from longer distances were mapped according to the localization of the counting point and assessed through the distance to the calling or drumming bird. During the field work, some cavities were found as seen in [19], and the centers of the territories were identified. Results of all inspections were drawn on the aggregate map, and based on the repeated observations, found occupied cavities, and spatial distribution of other observed birds, the territories of GHWs were designated. The number of breeding pairs in a given quadrat was determined based on directly heard or seen individuals. More than one territory was classified in the study plots only if more than two active woodpeckers were observed simultaneously. Approximately 50–60 days each year were spent on the fieldwork.

2.3. Data Collection

All 400 2 × 2 km squares were used in ArcMap to describe the habitat conditions based on data from the Forest Data Bank [23]. We calculated the database for each quadrant, containing the following parameters: share is the contribution percentage within each square (Table 1).

2.4. Statistical Analysis

We used the two best model parameters to assess the number of GHWs across the whole Augustów Forest area. This is essential information for the functioning of the nature conservation system and the designation of Nature 2000 areas. At the beginning of the analyzes, the correlations between variables were checked using the Spearman correlation test. The threshold for removing variables from the analyzes was set at R > 0.7. The results indicated that no correlations warranted the rejection of variables due to too high collinearity [24]. The highest correlation coefficient obtained for the two analyzed variables was 0.536 (Table A1).
We used generalized linear models (GLM) to model the dependencies between GHW distribution and habitat parameters. GLM was used to indicate the most critical factors affecting the abundance of GHW in the study plot. The number of GHW on each quadrant was the dependent variable, and all parameters in Table 1 were used as independent variables. The GLMs with a Poisson error distribution and a log link were used. To find the GLM that best fits the distribution data, we used the MuMin package [25] with the premise that the model could comprise up to 5 explanatory variables. The lme4 [26] library in R was used to construct the GLM. Statistical analyzes involving these variables and parameters were performed using R (version 4.3.3) statistical software [27].
The study area’s total population size was estimated using the “Predict” function in “R-Studio” [25]. We used a formula for the three best models found with the MuMin function, and data describing all 400 quadrants of Augustów Forest [25].

3. Results

The occupied territories of the GHW were not widely distributed. We found 28 active territories at 54 study plots. At a single counting point, zero to three GHWs were observed. The number of territories in a single quadrant ranged from 0 to 3; the most frequent were areas with no GHW (Figure 1). The overall density of GHWs in the study area was 0.13/km2, while at the optimal quadrats, it increased to about 0.75/km2.
Among the whole study area, there were preferred areas with a higher forest cover. It was verified how they differed from places without the occupied territory of the GHW. The binomial GLM results showed that the most important parameters varying quadrats with and without the GHW territories were the presence of watercourses, the share of mixed forests, and the proportion of fresh deciduous forests. The probability of the GHW’s nesting increased at plots with watercourses and a bigger share of mixed forest area. Deciduous forest proportion affected the model negatively but was insignificant (Table 2).
The best-founded GLM model results identified the same variables as key to assessing GHW abundance as a logistic model. The abundance of the GHW was positively correlated with the presence of watercourses (Figure A1), stands older than 120 years, a bigger alder share, and a bigger share of mixed forest area. The impact share of oak-dominated forests was negative but insignificant, only close to the significance threshold (Table 3). The next best models also indicated the share of the oldest forest, over 120 years, as an essential parameter for GHW number in the study area (Table A1). MuMin function results showed the next best models without considering the alder or mixed forest share (Table A1). The best model explained 55% of the variation, indicating a good fit. This model was also two times better than the next; its AIC was the lowest. We used the described model to assess GHW abundance based on these parameters.
The calculation for all 400 quadrats allowed us to estimate the population size at 160–200 breeding pairs. The model underestimated the number of quadrats with an abundance higher than more than one pair and overestimated the number of quadrats with 0.5–1 breeding pair territory (Figure 2). Our model predicted a lack of GHW in the 165 quadrats (41%) and in next 52 quadrats, less than 0.5 occupied territory. The spatial distribution is notable for large patches of forest where the GHW is not expected to occur. Non-occupied areas overlap the range of dry pine stands, while much higher abundances are predicted in the southern, wetter part of Augustów Forest (Figure 3 and Figure A1).

4. Discussion

Our study documented uneven distributions of GHW in the Augustów Forest. The probability of the GHW’s presence increased at plots with watercourses and a bigger share of mixed forest area. In turn, the abundance of the GHW was positively correlated with the presence of watercourses, stands older than 120 years, a bigger alder share, as well as mixed forest area. The impact share of oak-dominated forests was negative but insignificant. Next best models also indicated the share of the oldest forest, over 120 years, as an essential parameter for GHW number in the study area. Overall density was 0.13/km2. The population size was assessed at 160–200 breeding pairs.
The results we obtained confirmed the hypothesis of a positive relationship between the presence of the GHW and the length of watercourses and the presence of old stands. However, we did not find the expected positive relationship between the proportion of open areas and the presence of the GHW. In the Augustów Forest, occupied territories of GHW were distributed unevenly on the whole forest complex scale. The distribution may have been influenced by avoiding competition with the much rarer Green Woodpecker (Picus viridis), present mainly at forest edges and large arenas of open land around the villages [4]. The best explanatory predictors for the occurrence of the GHW among the variables analyzed were watercourses, a high proportion of mixed forest area, and a proportion of stands older than 120 years. The preference for watercourses and rivers in our study is linked to alders growing along water banks, sometimes even in small groups. Moreover, there are often small meadows near the water where the GHW can prey on ants. In turn, the proportion of alder was a positive predictor in the model for abundance assessment. This is consistent with our earlier research [19], in which we found that the alder was the most often chosen tree species by the GHW for cavity excavation in the Augustów Forest, but the most preferred tree species was the aspen [19]. Alder forest dominated near most watercourses, but not all alder stands were next to water. Thus, in the condition of the Augustów Forest, the proximity of watercourses provides GHW with breeding and foraging sites. Our observations show that along the main rivers in the Augustów Forest, the GHW is distributed regularly, with successive revetments every 1.0–1.2 km of the river. In contrast, we did not encounter the GHW in some, even large, patches of alder stands. This may be due to the lack of suitable feeding sites with low vegetation. Therefore, it appears that the watercourse explains the presence of GHW better than alder forests. There were partly similar results in the nearby Knyszyn Forest, where most revetments were in the ecotone zone, where swamp forests were adjacent to open areas, river valleys, or clearings [28].
Throughout its range, the GHW inhabits a variety of forest habitats in both lowland and mountainous areas. It prefers deciduous forests with beech, hornbeam (Carpinus betulus), oak, and aspen, as well as natural riparian forests [3,20,29,30]. In Rhodopes Muntains (Bulgaria), the GHW lives in diverse forest habitats but prefers stands with a participation of beech as well as pine. Most of the localities were in old forests with many dying or dry trees [31]. According to research by Domkos et al. [30] in Transylvania (Romania), the GHW nested mainly in oak and hornbeam forests with an admixture of aspen. The study by Südbeck [32] from Lower Saxony (Germany) have shown that the GHW inhabited beech and oak forests. Mixed forests with aspen are the primary habitat of GHW in northern Europe, in Norway and Finland [7,18]. Occasionally, the GHW occurs in modified areas such as orchards, tree rows, gardens, and cemeteries [3,17].
Recently, the GHW has been increasing its range due to a warming climate. Reduced snow cover depth increases winter survival due to a higher food availability [6,7,33]. The increase in abundance and expansion of the range of the GHW may affect the extension of the spectrum of habitats occupied. Likely, the higher proportion of mixed stands and coniferous stands in the forests of northern regions of the continent results in the use of this habitat type by the GHW, which, in turn, is rare in central and southern Europe.
In SE Poland, the territories of the GHW were present only in patches of alder stands [34]. A vital habitat factor for the GHW is the humidity of the sites and/or the presence of alder stands. Such observations align with the descriptions of habitat requirements by [3,17].
In our study, the proportion of mixed forest with spruce, pine, aspen, alder, and birch was a significant predictor. In the Augustów Forest conditions, those stands often grow on wet forest sites. According to Keller et al. [4], the GHW may occur in mixed forests with a significant proportion of conifers, provided old dead and dying trees are present and open areas are nearby. The proportion of spruce plantations up to 60% of the area did not reduce the probability of the presence of the GHW in Norway. The GHW may use mixed forest young plantations for feeding [3]. The spruce-dominated forest, the moist mixed forest, and the dry pine-dominated forest were the forest sites used by GHWs for nesting in southern Finland [19]. The woodpeckers selected their areas near the forest edge with large aspen trees [21]. Previously, our studies in the Augustów Forest showed that the GHW nests in four forest types, from mixed pine forest to deciduous forest and alder stands [19]. It would seem that the proportion of deciduous forests should positively affect the presence of the GHW, which was not confirmed by this study.
It is easy to explain the positive impact of the proportion of old-growth forests above 120 years old on the presence of the studied woodpecker. There are more dead and dying trees in older forests, which are the ones the GHW chooses to excavate for cavities [2,15,19]. Our previous research at the Augustów Forest [19] has shown that the GHW excavated cavities in trees aged 45 to 127 years. The average age of cavity trees was 83 years. All cavities were found in dying or dead trees [19]. Our present results show that the oldest forests benefit the GHW and other bird species. Other authors also highlighted the importance of old deciduous trees and old-growth forests for specialized forest bird species, e.g., [35]. In conclusion, the habitat requirements of the GHW in the Augustów Forest differ from those described in southern and western Europe [2], and are more similar to those described in Scandinavia [7,18].
In Poland, the GHW is not included in state bird monitoring. Therefore, data on long-term abundance trends of the species are lacking [11]. The Augustów Forest is an important Special Protection Area for Birds, where accurate GHW counts have been carried out in 20% of the area. Based on the data obtained in this study, the population size was estimated at 160–200 breeding pairs. The estimated population size in the Augustów Forest represents about 4.75% of the national population, assessed at 3000–5000 breeding pairs [10]. However, the actual abundance of the GHW in Poland is likely higher than reported by [10], even at 3600–11,000 pairs [9]. The reason for the differences in the assessment of the size of the national population is that there are not enough surveys aimed at assessing abundance by a uniform methodology on a representative number of sample plots, as elaborated by [17]. Earlier assessment for the Augustów Forest gave only 30–60 pairs [35]. Our survey was conducted in a region where the GHW occurred throughout the 20th century [29,36], but its abundance has probably increased recently. The several times higher abundance reported in this study is likely to be the result of accurate counts in the sample plots and an ongoing increase in the abundance of this species. A similar difference in population size was documented in the nearby Biebrza River Valley. The population size in 2012–2013 was estimated at 10–50 pairs, while in 2022 it was 114 breeding pairs, demonstrating a substantial increase in abundance [37]. Nevertheless, from the available information, it appears that in other forest complexes of northeastern Poland, the GHW population is lower than in the Augustów Forest. The population of the GHW from the Knyszyńska Forest was assessed at 25–40 breeding pairs and a density of 0.02/km2 [28], and from the Białowieża Forest at 30–35 pairs and a density of 0.05/km2 [38].
According to Wilk et al. [38], the Augustów Forest is one of the top 10 Nature 2000 SPAs where at least 1% of the national population of the GHW nests. In Poland, the most important refuges of the GHW lie in the south of the country, mainly in upland and mountainous areas and include beech forests admixed with silver fir (Abies alba): 250–300 pairs and an average density of 0.25/km2 in the Bieszczady Mountains, 250–300 pairs (density of 0.50/km2) in the Slony Mountains, 200–250 pairs (density of 0.34/km2) in the Przemyskie Foothills, 130–160 pairs (density of 0.10/km2) in the Beskid Niski Mountains, 50–60 pairs (density of 0.28/km2) in the Odrzańskie Forests, 35–50 pairs (density of 0.10/km2) in the Beskid Śląski Mountains, 40–60 pairs (density of 0.04/km2) in the Sandomierska Forest, and 63–90 pairs (density of 0.14/km2) in the Wałbrzysko-Kamiennogórskie Sudety Mountains [38]. Likely, changes in the abundance of the species are not as pronounced there as in NE Poland.
Our study describes the habitat requirements of the GHW in managed and partly protected areas of the Augustów Forest, studied during the breeding season. The GHW is a sedentary species outside the northern ends of its range, where it makes winter movements [7]. Numerous observations of individuals in the Augustów Forest in winter indicate that it is a sedentary species in the study area. However, our results may be subject to error due to the pseudo-replication of vocally active birds, as we did not determine the size or borders of home ranges, which can show considerable variability [9]. The population assessment may have been limited by the incomplete fit of the function used to estimate abundance. However, our results may be subject to error due to the pseudo-replication of vocally active birds, as we did not determine the size or borders of home ranges, which can show considerable variability [9]. The population assessment may have been limited by the incomplete fit of the function used to estimate abundance. The habitat preferences of the GHW might differ between seasons, but our study was conducted only during the breeding season. There are also some limitations (e.g. For example, the ability of birds to move to a voice source from outside their revirs) related on the playback experiments in data collections.

5. Conclusions

Our study indicates the most important parameters determining the presence of the GHW in the Augustów Forest, NE Poland. The woodpecker-occupied territory showed the strongest association with the length of watercourses, followed by a high proportion of mixed forests and the share of stands older than 120 years. The best habitat conditions for the GHW are created by alder woodland habitats along water courses, with some small open areas for feeding. The habitats used by the GHW during the breeding season in the Augustów Primeval Forest are more similar to the forests inhabited by this species in Scandinavia than in Central Europe. A practical indication for an inventory of breeding sites is to carry out searches along watercourses. To effectively protect the habitats of the GHW, it is necessary to maintain some areas of stands over 120 years old on wet sites. Old-growth forests are required because of dead and dying trees for cavity excavating. It is recommended to leave the green retention of old trees in managed forests as potential breeding places. In turn, clear-cut areas can provide foraging habitats. A GHW abundance assessment should be conducted on sample plots covering at least 20% of the big forest complex. In Polish forests, the abundance of this species is probably underestimated. There is a lack of complete data documenting the increase in the number in Poland.

Author Contributions

Conceptualization, G.Z. and D.Z.; methodology, G.Z.; software, G.Z.; validation, G.Z. and D.Z.; formal analysis, G.Z.; investigation, G.Z. and D.Z.; resources, D.Z. and G.Z.; data curation, D.Z. and G.Z.; writing—original draft preparation, D.Z. and G.Z.; writing—review and editing, D.Z. and G.Z.; visualization, G.Z.; supervision, D.Z. All authors have read and agreed to the published version of the manuscript.

Funding

This research received no external funding.

Institutional Review Board Statement

Not applicable.

Data Availability Statement

Data can be made available upon request and in consultation with the data owners.

Conflicts of Interest

The authors declare no conflicts of interest.

Appendix A

Table A1. A comparison of the parameters of the ten best-obtained GLMs for GHW abundance (ranked by AICc), Appendix.
Table A1. A comparison of the parameters of the ten best-obtained GLMs for GHW abundance (ranked by AICc), Appendix.
NumberInterceptWatercourses>120 ShareAlder ShareOak ShareMixed Forest ShareR2dfLog LinkAICcDeltaWeight
32−3.1842.0440.05710.0340−1.78700.08200.5046−35.2284.20.000.363
24−2.9072.0830.0469x−1.11400.10010.4665−37.1985.61.400.180
16−2.6901.8400.06390.0435−1.0830x0.4645−37.3085.91.630.161
30−2.3541.528x0.0262−1.31100.08870.4615−37.4786.21.960.136
22−2.3131.715xx−0.95330.10410.4324−38.8886.62.350.112
14−1.8161.387x0.0387−0.7502x0.3994−40.4189.65.410.024
12−2.5251.4320.05210.0372xx0.3754−41.4691.77.500.01
8−2.3312.0940.0571x−0.5531x0.3674−41.7992.48.170.01
29−1.511xx0.0441−0.91270.07080.3544−42.3593.59.290.003
10−1.8791.158x0.0359xx0.3233−43.5993.79.440.003

Appendix B

Birds 06 00032 g0a1
Figure A1. The predicted abundance of the GHW on the background of the distribution of old forests, watercourses, and water reservoirs in the Augustów Forest.
Figure A1. The predicted abundance of the GHW on the background of the distribution of old forests, watercourses, and water reservoirs in the Augustów Forest.
Birds 06 00032 g0a1

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Figure 1. The number of GHW territories in the study plots.
Figure 1. The number of GHW territories in the study plots.
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Figure 2. The predicted number of GHW territories in the study plots.
Figure 2. The predicted number of GHW territories in the study plots.
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Figure 3. The prediction of the abundance of grey-headed woodpeckers in the Augustów Forest, based on the best GLM parameters.
Figure 3. The prediction of the abundance of grey-headed woodpeckers in the Augustów Forest, based on the best GLM parameters.
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Table 1. The analyzed habitat parameters.
Table 1. The analyzed habitat parameters.
ParametersDescription
WatercourseThe presence of a river, stream, or canal
FieldThe presence of open areas, like fields or grasslands
LakeThe presence of lakes or ponds
TransportThe presence of a railway, an asphalt road, or a high-tension line
ForestShare of forest area
Fresh Pine ForestShare of pine-dominated forest at fresh, humid site types (percentage of deciduous trees 20–40%)
Wet/Bog Pine ForestShare of pine-dominated forest at wet and bog-humid site types (percentage of deciduous trees 20–40%)
Fresh Deciduous ForestShare of mixed and deciduous forest growing at fresh and wet site types (percentage of coniferous trees 20–40%)
Alder ForestsShare of alder forest at bog site types
Clear CutShare of clear cuts
DecayShare of stands over 140 years old with clear cuts, plus stands younger than 10 years old
Mixed-Aged ForestsShare of stands between 41 and 100 years old
Over 100 yearsShare of stands older than 100 years
Over 120 yearsShare of stands older than 120 years
Old Stand CompositionSpecies composition of old stands (pine/mixed coniferous/mixed coniferous and deciduous
Pine AreaShare of stands with pine domination
Mixed Forest AreaShare of stands with spruce domination, with a significant share of aspen (Populus tremula), alder, and birch (percentage of deciduous trees 30–60%)
Birch AreaShare of stands with birch domination
Alder AreaShare of stands with alder domination
Aspen AreaShare of stands with aspen domination
Oak AreaShare of stands with oak domination
Deciduous AreaShare of stands with deciduous tree species domination
Table 2. GLM results for relationships between the presence of the GHW and habitat parameters (binomial error distributed with logit link).
Table 2. GLM results for relationships between the presence of the GHW and habitat parameters (binomial error distributed with logit link).
Fixed Effect ParameterEstimateStd. Errorz-Valuep
Intercept−3.2451.02−3.1870.001
Watercourses3.0031.003.0010.003
Oak share−1.3650.99−1.3740.17
Mixed forest share0.3390.122.8290.005
Fresh deciduous forests share−0.1240.07−1.6990.089
AIC 48.459
Table 3. GLM results for relationships between the abundance of the grey-headed woodpecker and habitat parameters (Poisson error distributed with log link).
Table 3. GLM results for relationships between the abundance of the grey-headed woodpecker and habitat parameters (Poisson error distributed with log link).
Fixed Effect ParameterEstimateStd. Errort-Valuep
Intercept−3.1840.786−4.050<0.001
Watercourses2.0440.6373.2080.001
Oak share−1.7870.951−1.8780.06
Mixed forest share0.0820.0382.1390.03
Alder share0.0340.0172.0230.043
Over 120-year forests 0.0570.0272.1320.03
AIC 82.437
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Zawadzki, G.; Zawadzka, D. Habitat Requirements of the Grey-Headed Woodpecker in Lowland Areas of NE Poland: Evidence from the Playback Experiment. Birds 2025, 6, 32. https://doi.org/10.3390/birds6030032

AMA Style

Zawadzki G, Zawadzka D. Habitat Requirements of the Grey-Headed Woodpecker in Lowland Areas of NE Poland: Evidence from the Playback Experiment. Birds. 2025; 6(3):32. https://doi.org/10.3390/birds6030032

Chicago/Turabian Style

Zawadzki, Grzegorz, and Dorota Zawadzka. 2025. "Habitat Requirements of the Grey-Headed Woodpecker in Lowland Areas of NE Poland: Evidence from the Playback Experiment" Birds 6, no. 3: 32. https://doi.org/10.3390/birds6030032

APA Style

Zawadzki, G., & Zawadzka, D. (2025). Habitat Requirements of the Grey-Headed Woodpecker in Lowland Areas of NE Poland: Evidence from the Playback Experiment. Birds, 6(3), 32. https://doi.org/10.3390/birds6030032

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