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Acta Ornithologica (accepted July 2022)
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Do large herbivores have an important role in initiating tree cavities used by hole-
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nesting birds in European forests?
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Richard K Broughton 1, 2 *, Oliwia Karpińska 3, Katarzyna Kamionka-Kanclerska 3, Marta
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Maziarz 4
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1 UK Centre for Ecology & Hydrology, Maclean Building, Benson Lane, Crowmarsh Gifford,
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Wallingford, Oxfordshire OX10 8BB, UK. ORCiD: 0000-0002-6838-9628
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2 Senior Research Associate, Department of Biology, University of Oxford, Zoology Research
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and Administration Building, 11a Mansfield Road, Oxford OX1 3SZ, UK.
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3 Siedlce University of Natural Sciences and Humanities, Faculty of Exact and Natural
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Sciences, Institute of Biological Sciences, ul. B. Prusa 14, 08-110 Siedlce, Poland. ORCiD:
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Karpińska: 0000-0003-0926-3811; Kamionka-Kanclerska: 0000-0003-3493-7400
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4 Museum and Institute of Zoology, Polish Academy of Sciences, Wilcza 64, 00-679 Warsaw,
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Poland. ORCiD: 0000-0002-2921-5713
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Acta Ornithologica (accepted July 2022)
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ABSTRACT
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Tree cavities are an essential resource for cavity-dwelling mammals, birds, invertebrates and
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fungi, and so they are important for maintaining forest biodiversity. In North American forests,
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woodpeckers (Picidae) play a keystone role in cavity creation by excavating holes. However,
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in European forests many hole-nesting songbirds rely on non-excavated cavities that are
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formed by processes of fungal decay and compartmentalization after tree damage. Several
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factors are recognised in initiating non-excavated cavities that are used by hole-nesting birds,
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including loss of a tree branch or stem breakage, but this topic is poorly studied. Here, we
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propose that bark stripping by large herbivores (e.g. Red Deer Cervus elaphus and European
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Bison Bison bonasus) could be another important, and previously overlooked, mechanism for
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initiating tree cavities that are used by hole-nesting birds. We suggest that, after the initial
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damage from herbivore bark-stripping, fungal decay can create specific elongated, slit-like
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cavities, which are particularly important as nest sites for some common forest songbirds. We
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outline this idea using original observations and evidence from the literature, primarily from
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the primeval forest in Poland’s Białowieża National Park. We also use studies from elsewhere
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in Europe to show a generally low usage of slit cavities by birds where large herbivores are
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scarce or absent. We suggest that restoring such animals in European forests could help to
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restore the abundance and diversity of the tree cavity resource for hole-dwelling species. We
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encourage future research to investigate this proposal of large herbivores being important
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agents of tree cavity formation that could enhance biodiversity.
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INTRODUCTION
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Tree cavities are essential microhabitats in forest ecosystems, providing shelter and breeding
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sites for many mammals, birds, invertebrates and fungi (Stokland et al. 2012). Despite a large
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literature on cavity-using species, particularly birds and bats, there is no universal definition of
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a tree cavity, such as a typical origin or morphology. In the context of nesting birds, however,
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Acta Ornithologica (accepted July 2022)
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a tree cavity is broadly defined as a hole or hollow within a tree that provides a semi-enclosed
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chamber in which a bird can nest or roost, but these can take a variety of forms (Wesołowski
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& Martin 2017).
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Tree cavities are classified as excavated or non-excavated. Excavated cavities are created
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directly by animals, especially woodpeckers (Picidae), which peck or hammer into the wood
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to excavate a nest chamber (Wesołowski & Martin 2017). Non-excavated tree cavities are
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those formed by fungal decay of the wood following damage or infection, which can eventually
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result in a cavity (Stokland et al. 2012). Globally, tree cavities are more abundant in humid
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regions that promote fungal decay, although they appear to be less abundant in Palearctic
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forests, including those of Europe (Remm & Lõhmus 2011).
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Where forests have a history of management, as in Europe, there is typically a severe
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reduction in the availability of tree cavities, compared to natural forest (Walankiewicz et al.
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2014, Andersson et al. 2018). This cavity deficit is due to the logging of older and decaying
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trees, selective thinning, and the loss of keystone animals that create holes (Remm & Lõhmus
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2011). This reduced availability may limit the number and variety of hole-nesting animals that
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are reliant on pre-existing cavities.
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Providing artificial cavities, such as nest-boxes for birds or bats, is a common mitigation for
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the reduced availability of tree holes in managed woodlands. However, nest-boxes are costly
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and labour-intensive, requiring regular maintenance (Lindenmayer et al. 2009, Wesołowski &
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Martin 2017). Furthermore, nest-boxes have very different microclimatic conditions from
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natural tree cavities, which may have ecological implications, so they cannot be treated as
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direct substitutes for tree cavities (Wesołowski & Stańska 2001, Broughton et al. 2015,
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Maziarz et al. 2017).
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Due to a strong bias towards nest-boxes in ornithological research, however, studies of birds
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using tree cavities are particularly important for understanding natural behaviour (Wesołowski
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2011). In North American forests, woodpeckers perform a keystone role by excavating most
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Acta Ornithologica (accepted July 2022)
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of the tree cavities that are later used by other birds and mammals (Martin & Eadie 1999). In
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parts of Asia for where data are available, woodpeckers may be important but do not appear
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to have a keystone role for other cavity-nesting birds (Bai et al. 2005, Baral et al. 2018). In
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European forests, much of the literature on birds breeding in natural cavities comes from the
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long-running studies established by Ludwik Tomiałojć, Wiesław Walankiewicz and Tomasz
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Wesołowski, in Poland’s Białowieża National Park (NP). These studies suggested that
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woodpeckers are generally not keystone species in European forests (reviewed in Wesołowski
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et al. 2007a). Instead, non-excavated cavities, rather than old woodpecker holes, are far more
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important to hole-nesting songbirds in European forests than in North America (Tomiałojć et
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al. 1984, Wesołowski & Martin 2017).
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Different types of non-excavated cavities were outlined by Wesołowski (2007a) and
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Wesołowski & Martin (2017), including 1) ‘knotholes’ that form where a branch detaches from
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a main stem, 2) ‘chimneys’ that have an upward-facing entrance on snapped stems, 3) ‘cracks’
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formed by splits in the trunk, and 4) other ‘trunk holes’ in decaying wood, where a cavity forms
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on the main stem. Where trunk holes have an elongated, vertical opening, these are also
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referred to as ‘slit’ cavities (Wesołowski 2007a).
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There is little direct observation of exactly how non-excavated tree cavities are initiated, but
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they appear to form after damage to the protective bark through injury or disease, which
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exposes the underlying sapwood or heartwood (Stokland et al. 2012, Kõrkjas et al. 2021). A
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fungal infection can then take hold, leading to progressive decay of the wood, potentially
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leading to a cavity forming in the heartwood (Boddy 2001). A living tree counters the damage
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by compartmentalisation, which creates a protective callus around the wound or cavity (Shigo
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1984). Compartmentalisation in living trees is a dynamic process, changing the shape of a
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tree cavity and its opening over time, which may continue to expand and deepen, or the tree
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may heal over the entrance and the cavity becomes unusable (Wesołowski 2012).
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Wesołowski and Martin (2017) listed the ways that a non-excavated tree cavity could be
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initiated, which then develops to become usable by birds. These agents comprised snow,
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Acta Ornithologica (accepted July 2022)
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frost, fire, lightning, insects, foraging birds and the loss of branches. Kõrkjas et al. (2021) and
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Larrieu et al. (2018) added to this list of cavity-forming mechanisms with physical abrasion,
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caused by falling trees or rocks, and dendrotelms, which are natural basins in the tree that
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can deepen to hold water.
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Here, however, we propose another means of tree damage that appears to have been largely
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overlooked as a potential source of cavities that are important for hole-nesting birds, which is
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bark stripping by large mammals. The larger native herbivores of European forests, such as
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Red Deer Cervus elaphus, Eurasian Elk Alces alces and European Bison Bison bonasus, are
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known to habitually strip bark from young trees (deciduous and coniferous), predominantly in
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winter or early spring, by biting with their incisors and pulling off the bark in strips (Gill 1992a,
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Verheyden et al. 2006, Kelterborn et al. 2009, Paszkiewicz & Januszczak 2010, Welch & Scott
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2017, Kõrkjas et al. 2021, Jarnemo et al. 2022). Bark can also be damaged or stripped when
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these animals rub their antlers or horns on tree trunks (Gill 1992a, Edvardsson et al. 2021).
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Both activities result in exposed or damaged wood, and the opportunity for subsequent decay.
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Kõrkjas et al. (2021) and Gill (1992a, 1992b) reviewed how bark stripping and antler rubbing
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by large herbivores, particularly Red Deer and European Elk, exposed the underlying wood,
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and was one of the agents of damage and microhabitat creation on living trees. Edvardsson
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et al. (2021) and Paszkiewicz & Januszczak (2010) also showed how bark stripping and horn
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rubbing by European Bison left elongated wounds on trees that propagated decay, including
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stem rot, affecting up to 20% of available trees. However, despite the widespread
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documentation of tree wounds resulting from bark stripping, we can find no proposal by
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previous authors that this activity can lead to the formation of tree cavities that could be used
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by hole-nesting birds.
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Based on evidence from the natural forest of Białowieża NP, we propose that bark stripping
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by large herbivores may be an important agent in cavity formation, specifically of slit-like tree
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cavities that are important for communities of hole-nesting songbirds (Wesołowski 1989). To
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outline the theory, we first document examples of tree damage observed in the Białowieża NP
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Acta Ornithologica (accepted July 2022)
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that we attribute to large herbivores, showing an apparent progression of decay from initial
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damage towards slit-type cavities. We then highlight previous studies showing the importance
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of slit cavities for hole-nesting birds in natural forest ecosystems. We then review the
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prevalence of slit-like tree cavities in studies of hole-nesting birds from elsewhere in Europe,
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where large herbivores are depleted or absent. Finally, we ask whether restoring large
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herbivores to European forests through rewilding could re-establish an important component
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of woodland ecology, by promoting the abundance and diversity of tree cavities for hole-
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nesting species.
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TREE DAMAGE AND SLIT CAVITIES IN BIAŁOWIEŻA NATIONAL PARK
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The Białowieża NP is the principal site in Europe where the natural formation of tree cavities,
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and their use by nesting birds, can be observed in relatively undisturbed forest. The Białowieża
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NP is the last extensive remnant of primeval lowland temperate forest in Europe, containing
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an almost complete assemblage of native plants and animals in unmanaged old-growth
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stands. These stands have a multi-layered and multi-aged structure, consisting of up to a
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dozen tree species, with abundant decaying wood, where natural processes are little affected
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by direct human activity (Tomiałojć et al. 1984, Wesołowski 2007b). The few extirpated
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animals missing from the ecosystem are the Brown Bear Ursus arctos (though individuals
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sometimes occur: Diserens et al. 2020) and the globally extinct Aurochs Bos primigenius.
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Tomiałojć et al. (1984) and Wesołowski (2007a) listed up to 31 bird species that regularly
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breed in tree cavities in Białowieża NP, comprising nine primary cavity-excavators (including
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seven woodpecker species), 16 obligate secondary hole-nesters, and six facultative hole-
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nesters. As such, the tree cavity resource and hole-nesting community is virtually intact, in
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contrast to most degraded or secondary woodlands elsewhere in Europe that have reduced
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tree cavities and local extinctions of some cavity-nesting birds (Fuller et al. 2005, Broughton
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et al. 2013). As such, Białowieża NP is an invaluable reference site for exploring the
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Acta Ornithologica (accepted July 2022)
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relationships between species in forest ecosystems, including cavity resources and their
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users.
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We made three sets of preliminary observations of tree damage in Białowieża NP that we
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attributed to initial bark stripping (Table 1 and Fig. 1, with more example images in Appendix
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1). These observations involved the photographing of 81 trees that showed bark stripping
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injuries, elongated decay and slit-like cavities, which were opportunistically found during
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fieldwork at various locations across the forest in 2018 (see Broughton et al. 2020 for
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locations), largely within 187 ha of oak-lime-hornbeam Tilio-Carpinetum stands. In 2021, we
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made further observations of 26 trees showing recent bark stripping during vegetation
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sampling in the southern part of Białowieża NP (25 ha of Tilio-Carpinetum stands, within plot
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EN in Figure 1 of Broughton et al. 2020). Further sampling measurements and photographs
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of recent bark stripping and presumed subsequent decay, scars or slit-like cavities were taken
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from 34 trees in the same area of Białowieża NP in 2022 (Table 1, Fig. 1, Appendix 1).
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Sampling was not systematic, and so densities of trees featuring bark stripping cannot be
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inferred.
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We attributed these tree wounds to initial bark stripping by herbivores based on the height
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above the ground and the nature of the damage, the pattern of decay or scarring, and their
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apparent similarity to herbivore damage documented by Edvardsson et al. (2021),
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Paszkiewicz & Januszczak (2010) and Kõrkjas et al. (2021). The observations involved mostly
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young or thin trees, with a general diameter at breast height (DBH) of less of than 30 cm. The
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damage or decay was recorded between ground level and approximately 2.5 m high. Except
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for one Norway Spruce Picea abies, all of the trees involved were deciduous, mostly immature
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Small-leaved Limes Tilia cordata and Common Hornbeams Carpinus betulus, which are
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common in the area (Karpusta 2019), and also some Common Hazels Corylus avellana.
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Acta Ornithologica (accepted July 2022)
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TABLE 1. Attributes of sampled and observed trees in Białowieża National Park that exhibited marks,
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wounds, decay and cavities as a presumed result of bark-stripping by large herbivores.
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Sample
year
DBH
mean
cm
DBH
max
cm
Wound
height
m
Tree species %
Trees
n
Lime
Hornbeam
Hazel
Other
2018
-
~30
~2.5
33
63
0
2
81
2021
10
74
1.6
15
46
38
0
26
2022
17
57
2.3
58
30
12
0
34
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The three sets of observations in Table 1 are not systematic or fully representative. However,
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these observations are indicative of the kinds of tree wounds and decay on which we based
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our ideas, which is the proposed progression from initial bark stripping to subsequent decay
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and the eventual formation of some slit-like cavities that are used by hole-nesting birds.
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In our observations, fresh wounds from bark stripping (including horn or antler rubbing)
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appeared obvious and distinctive, consisting of damaged bark from gouging, fraying and
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removal of strips (Fig. 1A). The dark brown discolouration of exposed inner bark, fresh yellow-
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orange sapwood, and an absence of thickened scar tissue around the damage suggested that
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these wounds are relatively new, probably in recent weeks/months (Jarnemo et al. 2022).
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Wounds that appeared older were duller in colour and with thickened tissue surrounding
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exposed wood, as the tree compartmentalised the damage (Fig. 1B). Some wounds then
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clearly progressed to a deep elongated fissure within the bark, as it gradually closed over the
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wound (Fig. 1C). Wounds that had apparently closed over completely were indicated by a long
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scar on the bark (Fig. 1D).
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However, other wounds seem to progress on a different trajectory if the heartwood was
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degraded by fungal decay before the wound healed over (Shigo 1984). This decay appeared
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to create a cavity behind the exposed panel of wood, with an opening at the top or bottom of
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the fissure (Fig. 1E). Where the exposed wood had degraded completely, then the wound
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Acta Ornithologica (accepted July 2022)
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could become a narrow and hollow fissure within the tree, forming an elongated slit-shaped
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cavity (Fig. 1F). In other wounds, it appears that progressive decay of extensive damage may
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result in a large, open, elongated hollow (Fig. 1G).
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FIGURE 1. Wounds, scars and slit-like cavities on trees in Białowieża National Park, showing sequence
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of (A) initial bark stripping by a large herbivore causing an elongated wound; (B) older bark-stripping
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wound showing compartmentalisation (thickened callus) surrounding an exposed panel of heartwood;
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(C) wound healing to close over exposed heartwood and leaving a deep fissure; (D) successful healing
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Acta Ornithologica (accepted July 2022)
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that has closed over the wound and left an elongated scar; (E) failure of the wound to heal and decay
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of an exposed panel of heartwood, creating a cavity in a slit-like wound that is used by nesting Marsh
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Tits Poecile palustris; (F) total decay and disappearance of the exposed heartwood panel, creating an
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elongated slit-like cavity used by nesting Marsh Tits; (G) progressive heartwood decay that has formed
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a deep hollow in the tree.
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If the type of bark stripping and tree damage observed in Białowieża NP was indeed caused
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by large herbivores, the most likely species are Red Deer and European Bison. These species
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typically cause more extensive damage, affecting larger trees and reaching higher above the
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ground, compared to the smaller Roe Deer Capreolus capreolus (Gill et al. 2000, Paszkiewicz
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& Januszczak 2010). Eurasian Elk is relatively scarce in the forest (Jędrzejewski et al. 2002),
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and so its contribution was probably negligible. Obvious marks made by horns or antler tines
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are visible on some images, and could only have been made by large herbivores. There is
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currently no information for how long it takes for usable slit cavities to develop after the initial
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tree damage, as the trajectory and rate of cavity formation are poorly understood (Stokland et
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al. 2012). The typical lifespan of usable cavities in Białowieża NP is a median 11-13 years
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(Wesołowski 2012), suggesting that cavities may evolve on a timescale of years or decades
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in this forest.
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THE IMPORTANCE OF SLIT CAVITIES FOR HOLE-NESTING BIRDS
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The slit cavities depicted in Figs. 1E and 1F are distinctive among the types of tree holes
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documented in Białowieża NP. Kapusta (2019) sampled the tree cavities available to hole-
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nesting birds in Białowieża NP, and found that three-quarters of the cavity resource consisted
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of oval or elongated holes, with almost half of all cavities being strongly elongated or slit-like.
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Slit-like cavities also feature prominently in studies of nesting songbirds from Białowieża NP.
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Of the types of non-excavated tree cavity documented by Wesołowski (2007a) and
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Acta Ornithologica (accepted July 2022)
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Wesołowski & Martin (2017), it is the trunk hole cavities with elongated slit-like entrances that
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are particularly favoured by many of Białowieża NP’s hole-nesting birds. Wesołowski (1989)
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reported that 20% of all nests of seven hole-nesting species were in slit cavities. For several
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common species, including Blue Tits Cyanistes caeruleus, Great Tits Parus major, Marsh Tits
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Poecile palustris and European Robins Erithacus rubecula, slit cavities are the most frequent
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nest sites (Wesołowski 1996, Wesołowski & Rowiński 2012, Maziarz et al. 2015, Karpińska et
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al. 2022). Slit cavities are also commonly used by Collared Flycatchers Ficedula albicollis and
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Pied Flycatchers F. hypoleuca (Czeszczewik and Walankiewicz 2003, Walankiewicz et al.
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2007), and even by Common Blackbirds Turdus merula (Tomiałojć 1993), so they are clearly
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an important resource for hole-nesting songbirds in natural forests.
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The Białowieża studies show that many slit cavities used by nesting birds partially overlap with
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the height range of bark stripping by larger herbivores, i.e. within approximately 2 m above the
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ground (Gill 1992a, Paszkiewicz & Januszczak 2010, Vacek et al. 2020, Edvardsson et al.
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2021). For example, among nest sites of Great Tits and European Robins, a fifth of the slit
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cavities used by these birds, 20% and 19% respectively, were within 2 m of the ground
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(recalculated from raw data from Maziarz et al. (2015) and Karpińska et al. (2022)). The
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studies of nesting birds from Białowieża NP also show that those species nesting at lower
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average heights above the ground are more likely to use slit-like or elongated cavities than
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those typically nesting in cavities at greater heights (Fig. 2).
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These differences between species in cavity usage may be partially related to body size. The
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larger European Starlings Sturnus vulgaris and Eurasian Nuthatches Sitta europaea nest at
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the greatest mean heights and rarely use slit cavities (Fig. 2), preferring larger cavities with
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larger entrances, such as old woodpecker holes (Wesołowski & Rowiński 2004, Wesołowski
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2022). The larger Tawny Owl Strix aluco, which uses holes averaging 9.9 m above the ground
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in Białowieża NP, also does not use slit cavities, but this probably reflects their requirement
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for a much larger cavity entrance than any of the songbirds (Yatsiuk & Wesołowski 2020).
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FIGURE 2. Mean height of nest cavities above the ground versus the percentage of nest-cavities that
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are elongated slit-like cavities, for European Robin (R), Marsh Tit (MT), Blue Tit (BT), Great Tit (GT),
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Collared Flycatcher (CF), Pied Flycatcher (PF), European Starling (S) and Eurasian Nuthatch (NH). A
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linear regression trendline is shown (R2 = 0.32). Data are derived from studies in Białowieża National
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Park (see text).
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However, a declining frequency of slit cavities among songbird nest sites at increasing heights
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from the ground (Fig. 2) could, potentially, also reflect a declining availability of slit cavities at
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greater heights. The evidence to test this is lacking at present, and many slit-like cavities
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clearly occur at heights well above the range of bark stripping by herbivores. These higher
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slits must be caused by mechanisms other than herbivores, such as abrasion from falling trees
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(Kõrkjas et al. 2021) or initiated by bark stripping by Red Squirrels Sciurus vulgaris (Krauze-
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Gryz & Gryz 2015). As such, large herbivores cannot be the only initiators of slit cavities, even
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if they are potentially significant agents for initiating them closer to the ground.
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Nevertheless, slit cavities, including those within several metres of the ground, are clearly of
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great importance to nesting songbirds in Białowieża NP, particularly some of the tits,
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flycatchers and the European Robin. The frequent use of slit cavities by these species might
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reflect selection for their specific qualities as nest sites. Narrow entrances, with a minimum
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diameter of 16-30 mm, allows access to small songbirds but prevents medium-sized predators
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from entering nest cavities (Wesołowski 2002, Wesołowski & Rowiński 2012). Also, an
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elongated, slit-like entrance has a relatively large area, potentially improving gaseous
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exchange, and letting extra light inside the cavity (Wesołowski & Maziarz 2012, Maziarz &
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Wesołowski 2014); this would allow birds to nest at greater, safer distances from the entrance,
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beyond the reach of larger predators unable to fully enter cavities (Walankiewicz 1991,
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Wesołowski 2002). Consequently, slit cavities are generally safer nesting sites than other
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types of tree holes (Maziarz et al. 2016).
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EVIDENCE FROM TREE CAVITY STUDIES ELSEWHERE IN EUROPE
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If large herbivores play a significant role in initiating slit cavities used by nesting birds, as we
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propose, then these cavities should be less common in forests where large herbivores,
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especially Red Deer and European Bison, are rare or absent. The European Bison has been
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extinct across most of Europe for several centuries, whereas Red Deer have remained
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relatively widespread and abundant, but with a more fragmented or localised distribution in
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Scandinavia, western and southern Europe (Milner et al. 2006).
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In British deciduous woods where Red Deer are locally absent in many areas, and European
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Bison were never present (Kuemmerle et al. 2011), slit-shaped or elongated cavities (e.g. with
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a vertical dimension of the entrance that is at least twice as large as the horizontal) are indeed
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uncommon in studies of hole-nesting birds (Edington & Edington 1972, East & Perrins 1988,
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Broughton et al. 2011, 2015). Slits or elongated cavities are similarly scarce in studies from
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Germany (Ludescher 1973), Sweden (Nilsson 1984, Carlson et al. 1998), Estonia (Remm et
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al. 2006) and Iberia (Camprodon et al. 2008). In The Netherlands, Van Balen et al. (1982)
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found that cavities with oval entrances were the most common, although slits were present up
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to 2 m from the ground, and some could have been initiated by herbivores. However, it is
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unclear which herbivores were present in any of these European study areas.
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Importantly, though, based on the available information in the studies above, the proportions
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of elongated or slit cavities used by birds in these other European forests appear far lower
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than those reported from Białowieża NP, where slits account for a large number of nest
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cavities used by tits, flycatchers and robins. This supports the theory that herbivores could be
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a significant agent in initiating many slit cavities. Nevertheless, where slits are scarce, tits and
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flycatchers elsewhere in Europe are still able to nest commonly, even abundantly, in knotholes
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or other types of cavity outlined by Wesołowski (2007a) and Wesołowski & Martin (2017).
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Overall, however, in forests without large herbivores, fewer trees would be affected by bark
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stripping, and so probably fewer slit cavities would be expected to develop. As a result, the
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availability and attributes of the cavity resource may be more limited, such as the hole
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dimensions, height above the ground, predator defence, illumination and gaseous exchange
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(Wesołowski 1996, Wesołowski & Maziarz 2012, Maziarz et al. 2016). The studies from
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Białowieża NP show that, where slits are readily available, they are commonly or preferentially
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used by many songbirds (Wesołowski 1996, Wesołowski & Rowiński 2012, Maziarz et al.
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2015, Karpińska et al. 2022).
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SUMMARY AND FUTURE CONSIDERATIONS
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The evidence for large herbivores playing an important role as initiators of slit cavities is largely
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circumstantial at present. However, taken together, the theory seems persuasive, linking the
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bark stripping by large herbivores to tree wounds, decay and slit cavities, and then the
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extensive use of slit cavities by hole-nesting birds, as shown by the Białowieża NP studies.
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Further research is needed to provide the detailed evidence to underpin these proposed links.
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Such studies may extend to other regions, such as North America, where bark stripping by
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American Bison Bison bison or other large herbivores can also occur (Gill 1992a, Beschta et
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al. 2020). Establishing exactly how bark stripping might develop into slit cavities would be
332
valuable, including the frequency of stripping by different herbivores, which tree species are
333
most affected, and also the timescale of decay and cavity development, and how this fits
334
among other agents of cavity formation. Additionally, classifying cavity types in studies of
335
nesting birds or tree damage, similar to the nomenclature used by Wesołowski (2007a) and
336
Wesołowski & Martin (2017), would enable a more accurate assessment of the prevalence of
337
slit cavities and the role of herbivores.
338
Our theory at present is based on preliminary observations and the existing literature. A formal
339
study of the role of herbivores in tree cavity formation is yet to be undertaken, and the
340
information and hypotheses outlined here are aimed at promoting further research. If the
341
theory is correct, then the basic elements required for this mechanism of slit cavity
342
development would be the presence or large herbivores, sufficient time for the progress of
343
decay, and an absence of management interventions (such as selective thinning) that would
344
disrupt these natural processes.
345
Some of these elements are currently lacking in deciduous forests across much of Europe
346
because large mammals are depleted, management removes damaged trees, and logging
347
limits the timescale over which natural decay can operate. However, the reintroduction of large
348
native mammals and the restoration of natural processes and ecological complexity are
349
increasingly common approaches within the rewilding framework (Perino et al. 2019). There
350
is growing interest in trophic rewilding to restore large herbivores to parts of Europe from
351
where they had become extinct, including European Bison, Red Deer and Eurasian Elk (Tree
352
2017, Perino et al. 2019, Lord et al. 2020). Restoring these animals would also restore their
353
ecological functions in forest ecosystems (e.g. Nickell et al. 2018), which could include a
354
previously unappreciated role in initiating tree cavities, as we propose. Indeed, examination
355
by Edvardsson et al (2021) of preserved trees in early Holocene peat deposits in Sweden
356
suggests that there was frequent bark stripping by large herbivores in prehistory, leading to
357
the type of tree damage and subsequent decay that we observed in Białowieża NP.
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Acta Ornithologica (accepted July 2022)
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Meanwhile, reintroduction of European Bison to Damerower Werder and the Rothaar
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Mountains in Germany has resulted in frequent debarking of forest trees (Kelterborn et al.
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2009, Schröder et al. 2019), and reintroduction of Red Deer to Knepp Wildland in England
361
(Tree 2017) has also resulted in debarking and early cavity formation similar to that observed
362
in Białowieża NP (Appendix 2).
363
In conclusion, we propose that the potential role of large herbivores in increasing the
364
availability and variety of tree cavities is worthy of further exploration. The ecological function
365
of tree cavity initiation could provide further support for restoring large herbivores to European
366
forest ecosystems, with wider benefits for biodiversity.
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368
ACKNOWLEDGEMENTS
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We thank the Białowieża National Park authorities for access to the forest. We also thank
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Knepp Wildland, particularly Penny Green and Ivan de Klee. The authors owe a debt of
371
gratitude to Professors Ludwik Tomiałojć, Wiesław Walankiewicz and Tomasz Wesołowski,
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who provided valued knowledge, inspiration, support and friendship, and we dedicate this work
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to their memory.
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