Trees, and more specifically groups of trees, are of significant importance to avifauna. Their provisioning of food, either directly (fruits, nuts, blossom) or indirectly (attracting insects and other types of prey), in addition to their ability to act as a nesting site, roosting site, or otherwise, makes tree presence absolutely crucial to a successful and healthy bird population. Of course, different bird species will respond favourably to different tree species and stand structures, and this – amongst other aspects – is discussed below.
As alluded to above, the structure of a woodland stand will have a marked impact upon bird species present within a site. For example, active coppice woodlands will provide habitat to bird species not frequently (if at all) found in old-growth stands or even coppice of over 11-12 years since the last cycle (Fuller & Green, 1999), though wood pastures, forest glades, and even agricultural fields bordering woodland may provide niche habitat for particular birds, of which many may be associated with grasslands and the transitional zone (ecotone) between grassland and woodland (Costa et al., 2014; Hartel et al., 2014; Hinsley et al., 2015) – including the nightingale (Luscinia megarhynchos) and the chiffchaff (Phylloscopus collybita), in the UK.
Stand structure will also impact upon the growth of young chicks, with some species growing better in older stands and others in younger stands (Hinsley et al., 2002). This is due to some bird species feeding up in the crown of a tree, whilst others forage near to ground level. For ground foraging birds, there will likely be a lack food sources available, where canopy closure has occurred; as will there be a lack of ground-cover for nesting (Fuller & Green, 1999). Similar conditions can however be created by grazing mammals, with deer being a notable example in the UK and North America (Gill & Fuller, 2007; McShea & Rappole, 2000). Furthermore, ground-nesting and ground-foraging birds are also more sensitive to disturbance, and therefore their presence may also be limited in high-traffic areas and locations where predators (and herbivores – including deer and other grazing animals) are found in abundance (Ford et al., 2001; Fuller, 2001; Martin & McIntyre, 2007; Schmidt & Whelan, 1999). Vehicular traffic may also be an issue, and notably when a woodland site runs adjacent to a busy road (Reijnen et al., 1995). Research has therefore suggested that established woodland sites, free of major disturbance and possessing greater structural diversity than succeeding woodlands or coppiced woodlands, will provide for a greater array of bird species (Gil-Tena et al., 2007; Hinsley et al., 2009), though even amongst structurally similar habitats the species composition of a site may have a marked impact upon bird species diversity (Arnold, 1988).
In fact, a greater mix of tree species may bolster bird diversity, as was demonstrated by Díaz (2006) when bird species in pinewoods and oakwoods were found to be lower than in a stand containing both species. By a similar token, species composition may impact upon bird species that forage amongst foliage for arthropods and other food sources. Investigations by Robinson & Holmes (1984), for instance, demonstrated that the distribution of foliage within the crown of a tree will impact upon the foraging ability of particular birds; as will, but only at times, the size (and other characteristics) of foliage. Similarly, as particular tree species will attract certain arthropods, the species composition of a stand will impact upon the constituent bird species and their abundance. Thus, a mosaic of habitats that is mainly – but not at all exclusively – mature and mixed woodland may be most preferable if seeking to attract many species of bird. Such woodland need not be extensive in canopy cover however, as wood pastures attract such an abundance of insects that insectivorous birds can be found in great abundance, assuming the land is not treated with pesticides (Ceia & Ramos, 2016).
Building upon the concept of stand structure, the presence of standing deadwood is also important for birds. Whilst cycles of management are beneficial for some species, those that rely on old-growth stands with minimal management intervention are heavily reliant upon standing deadwood as a source of habitat (Drapeau et al., 2009). Those species which nest within recently-dead snags (or dead portions of living trees), including the woodpecker (Smith, 2007) – though also many species of secondary (successional) cavity-nesting species – will far more readily be found in stands of significant age that contain tracts of large (over 30cm DBH) potential habitat (Bednarz et al., 2004; Remm et al., 2006). Granted, not all standing deadwood is equal. For example, in the forests of British Columbia, USA, woodpeckers will preferentially frequent trembling aspen (Populus tremuloides), to the point that 95% of all cavity nests are found within this species – even in spite of its limited abundance within forest stands (Martin et al., 2004). Similarly, forest edge standing deadwood may be more preferable for some cavity-nesting birds (Remm et al., 2006), and at times standing deadwood created through recent forest fires may be most suitable (Nappi & Drapeau, 2011; Saab et al., 2004). Therefore, post-fire salvage logging may be detrimental to cavity-nesting birds (Hutto & Gallo, 2006). It should however be noted that not all cavity-nesting birds will create their own cavities from sites of decaying wood, and may instead use natural cavities that have formed at the branch junctions of snags (Remm et al., 2006).
The benefits of standing deadwood extend beyond the mere provisioning of viable nesting sites, however. They also act as suitable feeding platforms for many bird species, again including the woodpecker. In particular, decaying snags with lower wood densities will provide the suitable conditions for foraging (Farris et al., 2004; Weikel & Hayes, 1999). This is because such decaying snags attract saproxylic insects, which are viable sources of food for birds (Drapeau et al., 2009). However, this does not necessarily mean that such snags should be extensively degraded, as research has also suggested that snags with only some deterioration (through fungal decay and fire damage) are optimal for foraging (Nappi et al., 2003; Nappi et al., 2010). Without question, larger snags will normally provide for greater foraging potential, and not only because of the greater diversity of foraging site types (small branches, large branches, and the stem), but also because of the greater surface area upon which birds (including woodpeckers) may forage (Smith, 2007). By a similar token, snags can also be used for perching and communicating (Lohr et al., 2002), which could be of advantage to predatory birds and breeding birds, respectively.
Coarse woody debris (fallen deadwood) upon the woodland floor can also be of use to bird species. Lohr et al. (2002) identify such downed woody debris as being important for foraging, perching, and communicating; albeit at a generally lesser rate than standing deadwood (snags), though not always (Spetich et al., 1999). Understorey bird species may also utilise downed stems for nesting. Where coarse woody debris is removed therefore, bird species diversity and population abundance will almost certainly suffer (Riffell et al., 2011).
Of course, it is not only standing (snags) and fallen deadwood (coarse woody debris) that are of benefit, but also the decaying wood of living trees. Typically, it will be trees with more extensive internal decay and thus thinner strips of functional sapwood that will be more preferable to cavity-nesting birds (Losin et al., 2006). However, it is the larger individuals within a stand that will again be more readily frequented, with research by Conner et al. (1994) finding that the red-cockaded woodpecker (Leuconotopicus borealis) requires decaying heartwood of 15cm in diameter (or greater) to form a viable nesting site. Such extensive and suitable heartwood can usually only be found in older trees (Hooper et al., 1991), which therefore outlines the importance of conserving old-growth stands and retaining mature individuals during harvesting operations. In fact, red-cockaded woodpeckers will seek-out older trees wherever possible, because of the greater heartwood extent found within such trees (Rudolph & Conner, 1991).
Furthermore, akin to standing deadwood, not all trees are equal in their provisioning of viable habitat for cavity-nesting birds. Certain bird species may favour particular trees that are being decayed by specific heart-rotting fungi. Using the red-cockaded woodpecker as an example again, it is understood that Pinus spp. being decayed by the heart rot fungus Porodaedalea pini (syn: Phellinus pini) are highly desirable sites for nesting for the species (Jackson & Jackson, 2004). Similarly, the great spotted woodpecker (Dendrocopos major) will commonly frequent large oaks complete with large tracts of decaying heartwood and fungal sporophores (Pasinelli, 2007).
Birds may also utilise the tree’s flower (florivore), fruit (frugivore), and seed crops (granivore), as a source of food. In fact, birds are considered the most significant dispersal agent of a tree’s fruit and seed crops, which is testament to the important relationship birds and trees have in this regard (Howe & Primack, 1975; Sedgley & Griffin, 1989). Certain birds are even associated largely with specific tree species, such as how the Eurasian jay’s (Garrulus glandarius) main food source is the acorn of the oak (Quercus spp.) (Vera, 2000). Open-grown mature trees may typically harbour the greatest crops (Green, 2007), and parklands, pastures (Galindo-González et al., 2000), savannas (Dean et al., 1999), and even gardens and orchards (Genghini et al., 2006; Herzog et al., 2005) may be home to many such trees.
Unfortunately, pressure on these environments, be it in the form of grazing, chemical applications (particularly in orchards), or simply human activities, has led to declines in constituent bird populations, in some instances (Bishop et al., 2000; Elliott et al., 1994; Thiollay, 2006), though historically orchards amongst extensively-grazed wood pasture were highly valuable for bird species, which would feed upon the abundance of insects (Barnes & Williamson, 2011; Oppermann, 2014). Beyond the open-grown tree however, copses, woodlands, and great vast forests all have the ability to harbour birds, courtesy of their crops. Secondary and regenerating stands may perhaps provide for the greatest abundance and diversity of food for birds, given that the greater light levels provide suitable conditions for a wider range of plant and tree species that flower and subsequently produce fruits (Martin, 1985).
Additionally, the better light conditions mean such fruiting species are likely to be healthier and produce bigger and more plentiful fruits, which is of importance to foraging birds that seek out proteins, fats and carbohydrates from tree crops (Sedgley & Griffin, 1989) and insects attracted to flowers. One example of this would be how the plentiful silver birch (Betula pendula) stands, in Belfairs Wood (Essex, UK) during the 1970s, over-masted quite significantly and consequently attracted very large numbers of redpoll and finch (Carduelis spp.), which all foraged eagerly for the seed. By-and-large, as birds will seek-out fruits and seeds that are larger than average and in healthy supply upon a tree (Foster, 1990; Wheelwright, 1993), it is perhaps not surprising that such regenerating stands are highly desirable. Granted, closed-canopy and late-successional stands also harbour tree crops (including the acorns of Quercus spp. and keys of Fraxinus spp.) that are of huge value to birds (Greig-Smith & Wilson, 1985; Koenig & Heck, 1988). However, the poor soils (nutritionally and hydrologically) of many mature woodlands adjacent to agricultural landscapes had led to – at least in Australia – declines in fruit and seed crops and, as a result, bird population density (Watson, 2011).
Moving away from the woodland and forest stands, though not entirely returning to open-grown trees, we can observe how trees within field hedgerows can be of huge benefit to birds, as can trees within agricultural windbreaks. Benefit may come in the form of landscape connectivity, where hedgerows and windbreaks act as corridors connecting woodland patches to one-another (Davies & Pullin, 2007; Harvey, 2000; Leon & Harvey, 2006; Morelli, 2013), though they may also be used – albeit perhaps less frequently now, courtesy of increased hedgerow management (at least, in the UK) – as nesting sites and foraging sites (Benton et al., 2003; Netwon, 2004). Grass buffers either side of the hedgerow may aid with suitability for birds, as may the presence of a greater number of large trees within a hedgerow (Hinsley & Bellamy, 2000; Herzog et al., 2005).
Within urban environments, the presence of trees and hedgerows adjacent to busy roads can however have a negative impact upon birds, by increasing mortality rates (usually associated with birds flying out into oncoming traffic). Research by Orłowski (2008) concludes as such. Of course, the presence of trees is also of benefit, much like within farmland hedgerows. Urban street trees, and also those within gardens, can improve landscape connectivity, allowing for bird species to travel between more significant areas of tree cover found in parklands and urban woodlands (Sanesi et al., 2009). In particular, connectivity to older parks with remnant woodland fragments will support a greater diversity of bird species (Fernández‐Juricic, 2000). The advent of large coniferous tree (and hedge) planting in many urban areas, courtesy of the planting of the cypress and other conifers (including Chamaecyparia lawsoniana, Cupressus macrocarpa, and x Cupressocyparis leylandii), has also led to an increase in resident bird populations and primarily because of the over-winter shelter such coniferous tree species provide (Jokimäki & Suhonen, 1998; Melles et al., 2003; Rutz, 2008; Savard et al., 2000). Furthermore, sheltered trees within the urban landscape that have abundant fruit and seed crops can be of huge benefit to birds, by providing essential food sources in an otherwise somewhat undesirable landscape. For such reasons, urban parks and woodlands may potentially provide the best conditions for certain feeding birds, though large gardens complete with dense vegetation may also be of great importance. Tree-lined streets may also be critical, and notably so if trees are large, have dense crowns, and have an edible fruit or seed crop.
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