The Importance of the Scots Pine for the Diversity of Forest Avifauna: The Augustów Forest as a Case Study

Abstract

The Scots Pine Pinus sylvestris is the most widespread tree species in forests of lowland parts of Central and Eastern Europe. In Polish forests, stands dominated by the Scots pine cover almost 60% of the forest area. We analysed the relationship between forest birds and pine trees in an extensive forest complex in north-eastern (NE) Poland. Based on the 20-year-long study, we considered the percentage of broods established on pine trees and the percentage of Scots pine in stands on grouse leks and territory. A total number of over 20 bird species are dependent on the Scots pine trees or stands to locate their nests. Among this group, 11 bird species strongly associated with pine trees were confirmed, and they located at least 90% of broods or nests on pines. The Black Woodpecker Dryocopus martius selects the Scots pine to excavate cavities. Its cavities were regularly used by some species of secondary cavity nesters. In turn, the Common Raven Corvus corax nests were occupied by selected birds of prey. The importance of Scots pine for birds increased with the age of the trees. Old Scots pine trees were chosen by birds that built large nests, placed nests in the tree top, and excavated large cavities. Populations of pine-specialized bird species may be in danger due to the possibility of the Scots pine range moving to the north, which results from global warming. The maintenance of the Scots pine with the modification of the methods of forest regeneration should be included in the Polish forestry’s strategies and plans as a critical factor for the care of the population of rare breeding bird species.

1. Introduction

Because of its high ecological plasticity and use of a wide habitat gradient, the Scots pine Pinus sylvestris (hereafter SP) has a wide geographical range, including Scandinavia, Central and Eastern Europe, and a large part of Asia [1]. This is the crucial forest-forming species in Poland, growing in most site types. Stands of SP or featuring the species as the dominant one account for about 60% of the whole area of Poland under forest [2]. Furthermore, given the wide range of habitats occupied and high adaptability, the SP can survive in Poland’s lowland and upland forests of the coniferous or mixed/coniferous types [3]. Moreover, the species’ economic significance has been sufficient to ensure its introduction even where the site types are mixed/broadleaved and mesic broadleaved forest site types [1]. The SP often forms single-species stands or is a dominant tree species on poor sandy soils. It may be also a small admixture in stands growing on fertile soils. SP stands are used most often by clear-cutting [1]. The SP is one of Europe’s most crucial economic tree species [1,4]. This tree species’ physiology, pests, and diseases are deeply understood. Progressive global warming negatively affects pine forests due to water deficit (precipitation and groundwater levels) and rising temperatures [4,5]. According to a study of four European countries, average air temperature was the most critical factor influencing the radial growth of SP compared to precipitation totals. The SP thrives well in precipitation-stable locations. The study confirmed the fundamental effect of the ongoing global climate change on the dynamics and growth of SP forests in Europe [6]. Lowering the condition of Scots pine stands affects the increased activity of harmful insects (mainly beetles Scolitydae and moths Lepidoptera) as well as the mistletoe Viscum sp. mainly in the southern part of this tree species range [4,7].

Progressive climate change is altering the composition of trees in forests and reducing the number of species adapted to changing environmental conditions [8]. The considered scenarios of changes in the SP ranges due to climate warming predict the withdrawal of this tree species far to the north in the foreseeable future [9]. This is likely to result in changes in the species composition of forest communities in Poland and the whole of northern and eastern Europe, and consequently in the animal assemblages that inhabit them. The importance of the SP, in terms of its ecological role in the forest ecosystem, is rarely studied [10]. So far, the importance of SP trees for avifaunal biodiversity has yet to be analysed, as this tree species has been considered too widespread and thus has no impact on bird biodiversity [11]. The relationship of birds with other rarer tree species has been studied [12,13,14]. Climate change-related scenarios of a far northward shift in the European ranges of forest trees, including pine [9,15], as well as the deliberate reduction in the SP forest area by foresters, indicate the real prospect of threatening bird species strongly adapted to SP trees and the forest habitats they create. This threat mainly affects species ecologically specialised to the peculiarities of the SP stands as a result of long-term adaptation processes [16]. Changes in habitats affect the breeding habitats, including nesting places. Therefore, it is necessary to recognize the relationship of bird species with the SP.

Where the forests of Poland are concerned, some 100 species breed in pine stands or in those in which the species is at least present [17]. However, the coniferous-forest avifauna includes many species indeed associated with SP, as well as non-specialised generalists willing to use a wide range of habitats, and thus more of the high share of stands in general that the species takes are linked with the SP—rather than on account of any direct adaptation to this species of tree.

Thus, the work detailed here aims to point to those species of bird truly capable of being viewed as associated closely with pines as places to breed within Poland’s Puszcza Augustowska—the country’s largest contiguous area of forest, as well as to determine the nature of any dependent relationships. From the point of view of the conservation of biodiversity, it was kept in mind that bird species adapted to SP might soon face potential threats, given ongoing changes in climatic conditions, and hence these also determine site type and habitat. We hypothesized that the most strongly dependent are bird species associated with the mature phase of forest development.

2. Materials and Methods

2.1. Study Area

The Augustów Forest—located at 23°15′ E, 53°54′ N—is in the Polish province-region (voivodeship) of Podlaskie, on Poland’s borders with both Lithuania and Belarus and within the wider natural region of the Eastern Suwałki Lakeland and Augustów Plain. The Polish part is around 1140 km², while the forest complex is on a flat and sandy outwash plain dating back to the times of the Baltic Glaciation [18]. Where forest site types are concerned, it is a mesic coniferous forest that covers the most significant area (40%), followed by mesic mixed/coniferous forest (31%), mixed/broadleaved forest (6%), alder forest (5%), and moist mixed/coniferous forest (4%). Notwithstanding this diversity, the dominant species in most stands is Scots pine, and indeed, this species prevails over 78% of the entire forest area. In contrast, the black alder Alnus glutinosa is the typical species over just 9%, while Norway spruce Picea abies dominates at 8%, with birches Betula sp. at 5%, and pedunculate oak Quercus robur at 1%. The climate here is reflected in an average annual temperature of 6.6 °C, a growing season lasting some 190–195 days out of 365, and a snow cover that may persist for between 90 and 100 days. Prevalent winds are nevertheless from the west or northwest [19].

The northeastern part of the Augustów Forest includes the Lake Wigry National Park. The remaining parts of the forest complex are administered by the six forest districts of State Forests. Since 2004, Augustów Forest has come within the Natura 2000 network coded PLB200002, covering a total area of 134,377.72 ha. Subjects of protection within this area include no fewer than 41 species listed under the EU’s Birds Directive [20].

2.2. Field Study

The work detailed here was carried out in the years 2001–2023. An assessment of the level of dependence of different bird species in the Augustów Forest on the presence of the SP was done by reference to percentage shares regarding nests and broods on, in, or among SP. The study included birds of prey, woodpeckers, owl species, forest grouse, the common raven Corvus corax, and other selected non-Passeriformes birds. Only species for which there are representative data on at least 40% of breeding places of the entire area’s population were considered.

Bird species with at least 50% of their nests or broods established on SP trees or in pine-dominated forests were considered breeding-related. The relationship was assumed to be very strong (specialized species) with a proportion of at least 90% breeding on pine, strong (partially specialized species) with a proportion of nests at least 70%, and average with a proportion in the range of 51%–70% (species opportunistic to the SP). In the case of the ground-nesting forest grouse, the relationship was assessed based on the composition of stands in the leks (the Western Capercaillie Tetrao urogallus) or breeding territory (the Hazel Grouse Tetrastes bonasia), assuming very strong specialization in stands with at least 80% of SP trees, strong specialization in stands with 60%–80% of the SP, and opportunistic in stands with less than 60% of the SP. We used avian taxonomy after [21].

For bird species with body sizes at least 300 g nesting on trees, the assessment was based on the results of surveys conducted in the entire forest area, and for species with smaller body weights—in fifty 20-hectare representative sample plots, covering 1%–2% of the area of the whole forest complex.

2.3. Statistical Analysis

The Wilcoxon test was used to compare the proportion of the SP among nest trees, and the SP in nest stands. The Kruskal–Wallis test with post hoc Dunn test was used to compare differences in the proportion of the Scots pine in stands between developmental phases of the forest and the height of nest-located groups and nest size groups. Developmental phases were divided into 1–20 years old—young, 21–60 years old—middle-aged, 61–100 years old—matured stands, and >100 years old stands. The height of the nest located above the ground was divided into four classes: up to 15 m, 15–20 m, 21–25 m, and >25 m. Nest size was classified into four groups, too: 1—the largest (nest of the White-tailed Eagle Haliaeetus albicilla and the Black Stork Ciconia nigra), 2—big (nest of the Northern Goshawk Accipiter gentilis, the Eurasian Buzzard Buteo buteo, the Great Cormorant Phalacrocorax carbo), 3—medium (nest of the Grey Heron Ardea cinerea, the Western Honey Buzzard Pernis apivorus, the Short-toed Snake Eagle Circaetus gallicus, the Black Kite Milvus migans, the Red Kite Milvus milvus), 4—small (the Common Raven and the Eurasian Hobby Falco subbuteo). Logistic regression models (with a dichotomous variable in the form of nest on an SP tree (1) and not on an SP tree (0) were used to determine the importance/influence of tree age on the choice of the SP tree as a nesting site by all studied species—both birds building large arboreal nests and birds using cavities in SP trees. A separate logistic model was created to determine how the proportion of the SP in a stand affects the probability of choosing pine as a nest tree. Analyses and graphs were performed using the R environment and the R-Studio software (ver. 2024.04.01), using the packages dunn.test, sjplot, pastecs, ggplot2, ggpubr [22,23,24,25,26,27].

3. Results

3.1. Bird Species Associated with the Scots Pine

Together, 76 bird species were recorded on stands dominated by SP trees in Augustów Forest (Appendix A,

Table A1. Bird species observed during breeding season on the Scots pine stands study plots in the Augustów Forest. Abbreviations: Degree of migration: r—resident, m—migratory; Feeding guild: h—herbivorus, i—insectivorus, p—predator; Nesting place: g—ground, b—bush, c—crown, h—cavity.

Species	Degree of Migration	Feeding Guild	Nesting Place
Anas platyrhynchos	m	h	g, h
Bucephala clangula	m	i	h
Mergus merganser	m	p	h
Tetrastes bonasia	r	h	g
Tetrao urogallus	r	h	g
Columba oenas	m	h	h
Columba palumbus	m	h	c
Caprimulgus europaeus	m	i	g
Apus apus	m	i	h
Cuculus canorus	m	i	b
Scolopax rusticola	m	i	g
Ciconia nigra	m	p	c
Ardea cinerea	m	p	c
Phalacrocorax carbo	m	p	c
Pernis apivorus	m	i	c
Circaetus gallicus	m	p	c
Clanga pomarina	m	p	c
Accipiter nisus	r	p	c
Accipiter gentilis	r	p	c
Haliaeetus albicilla	r	p	c
Milvus migrans	m	p	c
Milvus milvus	m	p	c
Buteo buteo	r	p	c
Glaucidium passerinum	r	p	h
Aegolius funereus	r	p	h
Strix aluco	r	p	h
Upupa epops	m	i	h
Jynx torquilla	m	i	h
Dryocopus martius	r	i	h
Dendrocopos major	r	i, h	h
Dendrocoptes medius	r	i, h	h
Dendrocopos minor	r	i, h	h
Falco subbuteo	m	p	c
Oriolus oriolus	m	i	c
Lanius collurio	m	i	b
Garrulus glandarius	r	i, h	c
Nucifraga caryocatactes	r	i, h	c
Corvus corax	r	p, h	c
Prunella modularis	m	i	b
Anthus trivialis	m	i	g
Fringilla coelebs	m	I, h	c
Coccothraustes cocccothraustes	r	h	c
Pyrrhula pyrrhula	r	h	b
Loxia curvirostra	r	h	c
Carduelis spinus	r	h	c
Emberiza citrinella	r	i	g
Periparus ater	r	i, h	h
Lophophanes cristatus	r	i, h	h
Poecile palustris	r	i, h	h
Poecile montanus	r	i, h	h
Cyanistes caeruleus	r	i, h	h
Parus major	r	I, h	h
Lullula arborea	m	i	g
Hippolais icterina	m	i	b
Phylloscopus sybilatrix	m	i	g
Phylloscopus trochilus	m	i	g, b
Phylloscopus collybita	m	i	g, b
Sylvia atricapilla	m	i, h	b
Sylvia borin	m	i, h	b
Sylvia communis	m	i, h	b
Curruca curruca	m	i, h	b
Regulus regulus	r	i	c
Regulus ignicapillus	m	i	c
Certhia familiaris	r	i	h
Sitta europaea	r	i, h	h
Troglodytes troglodytes	m	i	g, b
Sturnus vulgaris	m	i, h	h
Muscicapa striata	m	i	h
Erithacus rubecula	m	i	g, h
Ficedula parva	m	i	h
Ficedula hypoleuca	m	i	h
Phoenicurus phoenicurus	m	i	h
Turdus viscivorus	r	i, h	c
Turdus philomelos	m	i, h	b
Turdus merula	m	i, h	b

). The collected data on the use of the SP trees for breeding by 19 bird species (

Table 1. Bird species nesting on SP trees. Preference: very strong—***, weak—*, lack of preference—0.

Bird Species	Number of Nests on the Scots Pine	Total Number of Nests	Percentage of Nests on the Scots Pine	Preference for the Scots Pine
Bucephala clangula	27	28	96	***
Columba oenas	244	248	98	***
Ciconia nigra	13	22	59	*
Ardea cinerea	66	67	99	***
Pharacrocorax carbo	170	290	59	*
Pernis apivorus	2	5	40	0
Accipiter gentilis	7	15	46	0
Circaetus gallicus	3	3	100	***
Haliaeetus albicilla	22	23	96	***
Milvus migrans	10	11	91	***
Milvus milvus	11	11	100	***
Buteo buteo	84	123	68	*
Clanga pomarina	1	10	10	0
Strix aluco	10	17	59	*
Aegolius funereus	28	29	97	***
Dryocopus martius	348	367	95	***
Dendrocopus major	42	150	28	0
Falco subbuteo	25	25	100	***
Corvus corax	120	121	99	***

) and on the species structure of stands in leks of the Western Capercaillie and Hazel Grouse territories were analysed (

Table 2. Ground-nesting bird species in stands dominated by the Scots pine. Preference: very strong—***, weak—0, *.

Species	Number (and Percent) of Leks, Home Ranges in Stands with a Share of the Scots Pine > 80%	Number (and Percent) of Leks, Home Ranges in Stands with a Share of the Scots Pine 60–80%	Number (and Percent) of Leks, Home Ranges in Stands with a Share of the Scots Pine > 60%	Preference for the Scots Pine
Tetrao urogallus	16 (100%)	0	0	***
Tetrastes bonasia	143 (52%)	81 (29%)	53 (19%)	*

), which accounted for 27.6% of all species observed. A total of 15 bird species established at least half of their broods on SP trees (Table 1). Resident bird species, i.e., those most strongly adapted to the forest environment, accounted for 40.9% of this group. During long-term surveys, more than 90% of nests or broods established on SP trees were documented for 11 bird species. Those closely associated with this tree species as a nesting tree included the Golden Eye Bucephala clangula, the Stock Dove Columba oenas, the Grey Heron, the White-tailed Eagle, the Short-toed Snake Eagle, the Black Kite, the Red Kite, the Tengmalm’s Owl Aegolius funereus, the Black Woodpecker Dryocopus martius, as well as the Eurasian Hobby and the Common Raven (Table 1). The proportion of the SP among nest trees of the studied bird species was significantly higher than the average proportion of this tree species in the surrounding nest stands (77% vs. 68%, U = 43, p = 0.038). Thus, the SP was chosen over its availability in the studied group of birds (except for the Lesser Spotted Eagle, the Western Honey Buzzard, the Northern Goshawk, the Eurasian Buzzard, and the Great Spotted Woodpecker).

Additional relationships were demonstrated among this group of specialised birds. The Black Woodpecker is a primary cavity excavator whose cavities were occupied by secondary cavity nesters: the Golden Eye, Stock Dove, and Tengmalm’s Owl. These birds were indirectly associated with the SP, showing a direct dependence on the availability of Black Woodpecker cavities, as these strongly preferred the SP as a nest tree (Figure 1). The second type of relationship involves birds using open nests. The Common Raven built nests on old SP trees. The Black Kite and the Eurasian Hobby regularly used nests of the raven for breeding (Figure 1).

An extreme dependence on SP stands has been shown for the Western Capercaillie and a weak one for the Hazel Grouse (Table 2). All of the Western Capercaillie’s leks were located in stands dominated by SP in over 80%, and one of the essential elements of the Western Capercaillie’s habitat was old, horizontally-branched SP trees on which the cocks display.

The other species under scrutiny showed weaker selectivity towards the SP: birds of prey—the Eurasian Buzzard, the Northern Goshawk, and the Black Stork. The proportion of nests on SP in these birds was similar to the proportion of this tree species in the stand in the study area. The pine-dominant stands contained 52% of the breeding territories of the Hazel Grouse. Only occasionally did the Western Honey Buzzard, the Lesser Spotted Eagle Clanga pomarina, and the Tawny Owl Strix aluco use pine for breeding. In addition to the Black Woodpecker, only the Great Spotted Woodpecker Dendrocopos major excavated cavities in the SP, but SP accounted for only 38% of this species’ nest trees. The other woodpecker species did not use the SP to create nesting sites.

3.2. Preferences for the Age and Proportion in the Stand of Scots Pine Trees

The particular species of bird associated with the SP used different developmental stages of stands. The largest species group (e.g., the Western Capercaillie, birds of prey, the Common Raven, the Black Woodpecker, and large secondary cavity-nesters) depended on old growth stages. The number of species associated with the SP increases with the age of the trees (Figure 2). This tree species reaches critical importance in older developmental stages and age classes.

The probability of choosing the SP by bird species for creating a breeding place increases along with the age of trees. The relationship is significant for all species strongly specialised for the SP, as well as for separately analysed nest sites of birds building large arboreal nests and birds nesting in cavities (Figure 3A–C,

Table 3. Logistic model results for the probability of choosing the Scots pine as a nesting tree.

		Estimate	SE	z Value	p (z)
All species	Intercept	−3.28	0.36	−9.17	<0.001
	Age	0.043	0.003	13.51	<0.001
Open nesters/crown nesting species	Intercept	−1.34	0.45	−2.97	0.003
	Age	0.022	0.004	5.60	<0.001
Cavity nesters	Intercept	−5.18	0.59	−8.65	<0.001
	Age	0.06	0.05	11.46	<0.001

).

The probability of selecting the SP by birds increased with the proportion of this tree species in the stand (Figure 4). More interesting, however, was that with single old SPs and an SP share of only 3%–5% among the stand, the SP accounted for about 40% of the selected trees among the nest trees. With a stand composed of about 50% of SP, the chance of the SP being chosen as a nesting site by the studied species accounted for more than 80% (Figure 4).

The SP proportion in pine-associated bird species’ nest stands did not differ significantly for the developmental stages of stands (KW = 6.858, df = 3, p = 0.08). The proportion of the SP in stands varied for bird species depending on the four nest size classes—higher for categories 2 and 4 and lower for categories 1 and 3 (KW = 29.81, df = 3, p < 0.001). Species with the largest nests (the Black Stork, the White-tailed Eagle—category 1), as well as the smallest nests (the Common Raven, the Eurasian Hobby—category 4), needed the oldest SP trees (KW = 83.538, df = 3, p < 0.001). The remaining bird species, with medium-sized nests, brought out broods on medium-aged trees (Figure 5A,B). Nests built 10–15 m above the ground were usually placed on trees around 100 years old, while nests built above 15 m were located on significantly older trees, i.e., those more than 120 years old (Figure 5C,D).

4. Discussion

4.1. The Importance of the Scots Pine Trees for Birds

Studying the relationship between the functioning of various elements of the forest ecosystem is essential, given the threat of climate warming-induced changes in the forest environment. The relationship with SP deserves attention because of its widespread distribution in the forests of Europe and the potential threat to this tree species [4]. Our study from the Augustów Forest showed an explicit specialisation towards the SP for 21 bird species, including a very strong relationship for 11 of them, nesting in crowns or cavities, or using them as lekking grounds. The presence of SP trees focuses on the possibility of reproduction of rare species, representatives of various ecological groups. SP trees are significant for resident birds, comprising 40% of this group. Ecological raptors accounted for almost 60% of associated SP birds, dominated by large-sized body species. Another distinct ecological group was cavity-nesters, accounting for 24% of the studied assemblage.

The actual number of pine-adapted species is likely higher. The study did not include several more birds associated with SP stands due to the under-sampling of found nests. Birds with a strong preference for SP stands also belong to the Woodlark Lullula arborea and the European Nightjar Caprimulgus europaeus, as demonstrated in other Poland forest complexes [28,29]. The relationship most often relates to a preference for the specific habitat created by pine stands on sandy soils. Other species associated with SP forests, although not always nesting directly on SP trees, are those belonging to the Passeriformes: the Common Redstart Phoenicurus phoenicurus, Eurasian Treeceeper Certhia familiaris, Coal Tit Periparus ater, European Crested Tit Lophophanes cristatus, European Robin Erithacus rubecula, Common Goldcrest Regulus regulus, European Pied Flycatcher Ficedula hypoleuca, Song Thrush Turdus philomelos, Mistle Thrush Turdus viscivorus, and others [30]. In the Augustów Forest, at least 60 species of breeding birds were found in SP-dominated stands in coniferous, SP-dominated habitats [31]. Still, the occurrence of only some of them depended on the SP, while others treated the SP opportunistically.

Our study shows additional interspecies relationships. Among the primary cavity excavators, the Black Woodpecker was the most important ecologically as a species with a strong preference for the SP while also creating nesting sites for several secondary cavity nesters. The presence and high abundance of this woodpecker species determine the formation of subsequent relationships with the SP in the breeding bird assemblage [32]. The Common Raven, as a species with a strong preference for the SP, creates nesting sites for two species of birds of prey. The Eurasian Hobby nested almost exclusively in the nests of the Common Raven, while the Black Kite located about two-thirds of its broods there. Therefore, both of these species of birds of prey were directly related to the availability of the breeding site of the Common Raven, strongly choosing the SP as a nest tree, and only indirectly to this tree species. However, the Black Kite built one-third of the nests independently. Although the Common Raven builds most of its nests on SP trees throughout its lowland range, it has nowhere been as strongly dependent on this tree species as in the Augustów Forest [33]. In contrast, the White-tailed Eagle chooses the SP most often, accounting for about 70% of its nest trees throughout Poland; however, its preference for the SP in the Augustów Forest (above 95%) was thus stronger than in the whole country [34]. The Western Capercaillie is the species most closely associated with SP trees, providing proper habitat structure, places for lekking, rearing young and resting, and food throughout the annual cycle [35]. A severe decline in the numbers of the Western Capercaillie caused by eutrophication and climate change has been documented in the Augustów Forest. The presence of spruces in the undergrowth or underbrush negatively affected the probability of locating leks, while an increase in the proportion of SP above 90% raised this probability [36].

On the other hand, not all bird species for which the SP was the most abundant nest tree species can be considered as those preferring it, given the high share of the SP in stands. A weak dependence on the SP as a nest tree for Eurasian Buzzards and Northern Goshawks was indicated by earlier studies in the northern part of the Augustów Forest [33]. The proportion of Black Stork nests located on the SP indicates the frequent use of this tree species for nest placement. However, a detailed analysis of the Black Stork’s nesting preferences in the Augustów Forest showed that the oak, rather than the SP, was the most strongly preferred tree species. The SP can be described as a second-choice nest tree for the Black Stork [34]. The Hazel Grouse was most numerous in forests with a high proportion of the Norway Spruces, showing a preference for the presence of this species in the stand. In single-species SP stands, it was less numerous than in mixed forests [37].

Data from other surveyed pine-dominated forest complexes in the east, as well as northern Europe, indicate a similar set of bird species. Still, there is a lack of studies showing direct links to SP trees and the use of this species for nesting. Interestingly, bird assemblages dominated by Passerines were similar in study plots from Poland and Finland [38]. In the SP forests of western Belarus, the highest number of species was recorded in the oldest forests, including the Black Woodpecker, Common Raven, Black Stork, Northern Goshawk, Eurasian Buzzard, and Eurasian Hobby, among others [39].

4.2. Features of Scots Pine Trees That Are Important to Birds

The older the stand means the stronger the birds’ dependence on SP trees. Those large body-sized species that build the largest nests (the White-tailed Eagle and the Black Stork) and the species building the highest-placed nests (the Common Raven and the Eurasian Hobby) need the oldest SP trees. In contrast, birds that build medium-sized nests are less dependent on the availability of Scot pine trees and can fulfil their nesting needs on trees of other species. Thus, the presence of old SP trees is crucial for birds that build the largest nests.

Birds’ preference for the SP is due to the specific features of mature trees of this species. The straight, high-cleared trunk gives the possibility of excavating large cavities on the high level above the ground, and the construction of the crown of old trees with thick branches allows the placement of even the largest nests, such as those of the White-tailed Eagle and the Black Stork [34]. Because of its suitable structure for excavating cavities, the SP is a key species for some primary and secondary cavity nesters, especially those of large body sizes. Moreover, data from the Augustów Forest show that the largest excavator, i.e., the Black Woodpecker, located more than 90% of its cavities in old SP [32]. This is a characteristic feature of forests with a low proportion of deciduous trees.

Old SP stands with loose crown density, and horizontal branches, provide the Western Capercaillie with places to rest and perch. Moreover, the plant species of shrubs associated with SP trees—e.g., berries Ericaceae—form the basis of the forest grouse diet during the vegetation season [35]. SP trees form specific plant communities, dominated by blueberry shrubs Vaccinium sp., with characteristic flora and fauna [10]. Stands dominated by the SP create a forest habitat with many insect species that provide food for numerous insectivorous birds. This, in turn, determines the nesting possibility of many bird species here, mainly Passeriformes, not only on SP trees but also on the ground or bushes [31]. This study emphasized the importance of the SP for birds during the breeding season but did not consider the importance of pine stands in post-nesting and the winter season. During these periods, additional species, including Great Spotted Woodpeckers and numerous Passeriformes, use the SP stands for foraging, e.g., the Northern Long-tailed Tit Aegithalos caudatus and Red Crossbill Loxia curvirostra [30].

4.3. Protective Recommendations

The SP tree is a locally important tree species, conditioning biodiversity protection. Our research shows that, with age, pine trees are of increasing ecological importance as a nesting site. It is necessary to protect old pine stands and pine outages in mixed stands because old SPs are preferred nesting sites by various species that build large arboreal nests and use large arboreal cavities. However, according to published forecasts, in the next 50 years, the SP may disappear from Polish forests due to changes in climatic conditions [9]. Pressure from insect pests, fungal diseases, and die-offs due to water stress is increasing [4,40]. The deteriorating condition and health of SP forests in Central Europe makes foresters wary of using pine trees extensively in forest restoration. However, the vulnerability of the SP to mortality depends on local conditions and how stands are regenerated and managed [41]. Based on our results—as well as the prospects for the functioning of pine stands—it can be expected that the decline in the proportion of SP in stands in the long term will reduce the population of SP-dependent bird species.

In forest management, it is necessary to preserve old stands of SP trees as well as to restore the areas with the SP used in cutting. It is recommended to diversify silvicultural methods for SP stands, e.g., by using close-to-nature silviculture, which leads to the formation of multi-layer and multi-generation stands [42,43,44]. In this regard, preserved patches of SP forests can be refugia for the existence of many boreal and subboreal species much further south of their primary habitats. In turn, in more fertile forests, habitats should be ensured a small proportion of the SP in the regeneration phase [45].

5. Conclusions

For several thousand years, SP trees have remained a significant component of forest ecosystems in the central and eastern parts of the lowland areas of Europe. The habitat preferences of numerous bird species, typical of Poland’s lowland forests, are the result of their natural evolutionary adaptation to building nests and cavities in the trees of this species, as well as to using the plant communities created by the SP. The most important bird species strongly associated with SP trees include the Western Capercaillie, the White-tailed Eagle, the Black Kite, the Common Raven, the Eurasian Hobby, and the Black Woodpecker, as well as cavity- and nest-dependent secondary cavity-nesters. Most bird species associated with the SP require the oldest trees of this tree species.

The scenario for SP trees’ range is changing due to global warming, which is likely to affect the loss of breeding habitat for birds that are highly specialised in nesting on SP trees or in coniferous sites. For their protection, it is necessary to ensure the continuity of this tree species and a certain proportion of pine stands in Polish forests. To this end, new silvicultural methods should be used, allowing the formation of more structurally and species-diverse stands. Maintaining a certain amount of pine stands with the protection of the oldest trees should be considered in the strategies and plans of Polish forestry as a key to preserving the population of forest breeding bird species in Poland.