We used two study sites: the human-dominated Mwekera National Forests No. 6 in Kitwe, and the undisturbed forests of Chimfushi Wildlife Orphanage in Chingola, both in the Copperbelt province of Zambia (Figure 1) (hereafter referred to as Mwekera and Chimfushi). Miombo woodlands cover ~45% of Zambia’s landmass (Stringer et al. 2012) and in both study areas, the vegetation is predominantly miombo, characterised by vegetation types in Table 3. In the case of Mwekera, there are also exotic Eucalyptus globulus (blue gum), Eucalyptus tereticornis (forest red gum), Pinus kesiya (khasi), Pinus oorcarpa (ocote chino) and Pinus merkusii (merkus) plantations.

We conducted bird surveys at the two sites during August and September 2014 for owls and in November–February 2014/2015 for the rest of the cavity-nesting birds. These periods coincided with the time when most of the tree cavity-nesting birds such as southern ground hornbills, barbets, hornbills, and woodpeckers have their breeding seasons (Hockey et al. 2005). Observations were conducted during 09:00–11:00 hrs when birds were typically active. Data was collected from ten randomly selected sampling plots of quadrats that measured 200 m x 200 m in size, and were located at least 500 m away from vegetation ecotones. The plots were visited and inspected once every fortnight during the survey period. Although the number of bird species was not enumerated in this study, data on species presence and absence were recorded. Two observers searched each plot for tree cavities and dead wood. The use of cavities by birds was ascertained by remote observations of bird nesting, feeding, and social behaviour around the nesting sites (Dowsett et al. 2008; Leonard 2005; Newman & Read 2008). By using this indirect method, we did not need to reach and inspect physical features such as remnants of eggshells, feathers and droppings in the nests, thereby minimising disturbances to the cavity-nesting birds. Therefore, consistent bird activity in the form of nesting, feeding and roosting at the nest sites during the breadth of study period (or lack thereof) was taken as being indicative of whether the respective nests were still active or abandoned. Placement of nests was indirectly measured from tree DBH approximately 1.3 m above the ground to determine the tree size preferences by cavity-nesting birds and angular measurements for estimation of tree heights were made by use of Suunto hypsometer clinometers, following standard methods described by Avery and Burkhart (1994) and Leverett (2010). Hundred-metre measuring steel tape and rope were also used for estimation of distances. Enumerated trees were marked and tagged with metal plates for ease of subsequent identification. Differences in tree diameters over study period were not obtained on assumption that in-season variations resulting from contraction and expansion of trees were insignificant during the relatively short peak season for the breeding cavity-nesting birds as suggested by Krauss et al. (2007) and O’Brien et al. (2008). Dried and rotting wood was considered dead; wood in a posture perpendicular to flat ground (with about zero slope) was taken as standing wood, while wood prostrating on the ground was considered ‘lying’ wood. Tasco binoculars, with magnification of 20×, were used to aid in identifying cavity-nesting birds. Guide books were used for taxonomic classification of plants and birds (Leonard 2005; Newman & Read 2008).

Minitab (2004), (Minitab Inc., Pennsylvania, USA), version 14 statistical software was used in the statistical analyses. Two-sample T-test was used to establish whether the quantum of trees and active cavities significantly varied or not between the two study sites, using the procedure stipulated by Fowler, Cohen and Jarvis (2006). Relative plant species abundance was analysed for comparative commonness or rarity in the two study sites (McGill et al. 2007). Since diversity is non-linear, beta diversity which compares elements of diversity such as different vegetation communities was adopted. Thus, we derived comparable estimates of the effective number of tree cavities for bird nesting from exponentials of Shannon-Weaver indices (Jost 2006; Magurran 2004). From conservation perspective, Shannon-Weaver index is used to measure and compare components of conservation status between different sites with similar environments (Fowler et al. 2006). It is represented by H′ = −ΣP_i(lnP_i), where H′ is diversity index, P_i is the proportion of each component in the sample and lnP_i = natural logarithm of this proportion. The higher the H′ value, the greater the diversity. Use of indices is important and common in biodiversity monitoring due to the difficult in elucidating absolute enumeration of biodiversity (Barry et al. 2013; Tuomisto 2012). In the case of bird abundance, presence or absence data were analysed for each cavity-nesting bird.

There were 41 cavity-nesting bird species at the two study sites (Table 1). Cavity-nesting birds with the highest number of species were owls (seven species), woodpeckers (six species) and kingfishers (six species) and the birds with the lowest number of species were chats and parrots (Table 1). Out of the 41 bird species in the study sites, only 12.2% (n = 5) occurred in human-dominated forests (Table 1).

The undisturbed and disturbed forests had mean tree densities of 3.2 ± 0.02 trees per ha (range: 2–5 trees per ha) and 1.8 ± 0.05 trees per ha (range: 1–2 trees per ha) respectively. Mean tree densities in undisturbed forests were significantly higher than in disturbed forests (T-test=5.52; df = 9; P < 0.001). The mean abundance of active cavities was higher in the undisturbed site relative to disturbed sites at 17.4 ± 2.4 (range: 11–26) and 4.0 ± 1.1 (range: 1–7) (T-test = 12.79; df = 9; P < 0.001, Table 2). The number of tree cavities in undisturbed forests was 139.6% greater than in disturbed forests out of the 360 tree cavities that were recorded in the study area. Similarly, the number of tree cavities with active bird presence was 321.9% greater in the undisturbed forests than disturbed forests. The remainder of the 146 cavities in the study area were abandoned by the cavity-nesting birds. There were 24.6% fewer abandoned cavities in undisturbed than in disturbed forests (Table 2). The rate of cavity abandonment was about twice as high in human-dominated forests compared to undisturbed forests (61.3% c.f. 31.9%) (Tables 2 and 3).

Undisturbed forests had 71.9% more dead wood than human-dominated forests, as recorded from a total of 14 tree species and 201 dead wood (Table 2). More than two thirds of dead wood with cavities was found in undisturbed forests. There were 44.8% more wood without cavities in human-dominated forests relative to undisturbed forests.

Birds placed their cavity-nests at greater heights in human-dominated forests than in undisturbed forests (Table 2). Out of the 201 recordings of dead wood, 81 were prostrate while 123 were standing: a greater proportion of standing wood was found in undisturbed forests (Table 2). Prostrate wood in human-dominated forests included those that were cut by humans and left to dry. Diameter at breast height for the trees used by cavity-nesting birds ranged from 31.2 cm to 39.2 cm while cavity-nest height ranged from 3.2 m to 5.7 m above the ground (Table 2). Although undisturbed forests had suitable trees (> 36.0 cm tree DBH and > 5.0 m high) for cavity-nesting birds, a substantial proportion of nests (48.3%) were also placed in small trees (< 36.0 cm DBH and < 5.0 m high). In contrast, a small proportion of only 20% of the total cavity-nests were placed < 5.0 m in trees of < 36.0 cm DBH in human-dominated forests. In the entire study area, the cavity-nesting birds preferred large tall trees for nesting as depicted in Figures 2 and 3. The effective number of active cavities was almost three times greater in undisturbed forests than in the human-dominated forests, indicating enhanced nesting sites in the former (Table 2).

The tree species richness was higher in undisturbed forests (inverse of Simpson’s richness index = 12.6) than in human-dominated forests (inverse of Simpson’s richness index = 10.0). Further, cavity-nesting birds showed varied preferences for host trees (Tables 3 and 4). B. spiciformis, J. paniculata, P. curatellifolia and Uapaca kirkiana hosted a greater variety of cavity-nesting birds than other host trees. Arnot’s Chats had a wider selection of host trees for cavity-nesting than other cavity-nesting birds.

In this study, undisturbed forests found in the Chimfushi area supported a more diverse array of cavity-nesting birds than human-dominated forests in Mwekera. In both forest types, environmental conditions were similar except for the direct anthropogenic factors such as uncontrolled firewood collection, logging, wild fires, and land clearing for agriculture purposes that occurred in the Mwekera forests. The aforementioned anthropogenic factors could have been responsible for the occurrence of much less abundance and diversity of cavity-nesting birds in the Mwekera forests than the Chimfushi forests. According to Trzcinski, Fahrig and Merriam (1999) and Roberts (2007), environmental factors such as deforestation and habitat fragmentation might trigger the presence or absence of certain cavity-nesting birds in a particular area. Based on studies conducted elsewhere, it would also appear that undisturbed forests contain more cavity-nesting birds than disturbed forests, due in part to increased availability of nesting sites (dos Anjos 2006; Reid et al. 2014). Some bird species in the study areas might have been affected by direct persecution by humans and other predators. For instance, owls are persecuted by local communities based on superstitions surrounding them (Lee & Irwin 2005), despite owls being natural biological control agents of pests such as rodents and insects in agrarian and semi-urban landscapes (Meyer 2008).

In both study areas, the most prevalent cavity-nesting birds were the African Scops Owl and the Wood Owl and these birds had successfully adapted thus far to the human-dominated landscapes. There is the likelihood that both the African Scops Owl and Wood Owl species persist on the abundant rodents and insects found in human-dominated landscapes (Carrete et al. 2010; Dowsett et al. 2008). However, the quantity and diversity of bird species any forest can support depend on the capacity of the vegetation to absorb disturbance (Carpenter & Brock 2008; Folke et al. 2010). Cavity-nesting bird populations are largely resident and non-migratory in nature (Reid et al. 2014; Wilson & Hockey 2013). On the other hand, secondary cavity-nesting birds such as southern ground hornbills, Arnot’s Chats, and White-faced Scops Owls are likely to be more negatively impacted by the absence of the host vegetation than primary cavity-nesting birds that create their own tree cavities, without having to adopt or re-use the existing cavities. The dependence on existing cavities by secondary cavity-nesting birds speculates why secondary cavity-nesting birds have difficulties to survive forest disturbance trajectories such as forest succession and transformations (Martin & Eadie 1999). In addition, cavity dwelling bird species such as Bearded Woodpecker (Thripias namaquus), Olive Woodpecker (Mesopicos griseocephalus), Cardinal Woodpecker (Dendropicos fuscescens) and Golden-tailed Woodpecker (Campethera abingoni) implement various survival strategies against predation risks, including predator avoidance and narrowing sizes of entrance holes (Kemp 2001; Reed 2001). According to Sinclair, Fryxell and Caughley (2006), the ability for birds to adapt to environmental changes through use of rare resources by generalist foraging and nesting behaviour in a range of habitat conditions may contribute to the persistence of particular avifauna.

The quantity of suitable wood for nesting cavities is typically a limiting factor for populations of cavity-nesting birds (Laudenslayer 2002; Newton 1998). Adverse anthropogenic activities such as land clearing for agriculture, logging, uncontrolled fires and firewood collection can reduce the number of cavities below a critical threshold for the persistence of cavity-nesting birds. Human-induced and uncontrolled bush fires in the miombo woodlands are common, especially in Zambia where implementation of forestry policies and laws are relaxed (Eriksen 2007). However, factors such as the abundance of live or dead, rotten or solid, broken or intact wood, and also the presence or absence of certain wood types are particularly important in determining the persistence of cavity-nesting birds.

In this study, 81.3% of active nests were found placed high on large-diameter trees (Figures 2 and 3). The selection of large-diameter trees in human-dominated forests indicated that there was a preference by birds to use of large-diameter nest trees over small ones. The reason cavity-nesting birds preferred large-diameter trees could have been that birds enjoyed protection from predation, competition and some human disturbances, maximised nest space and thermal insulation (Laudenslayer 2002). For instance, nests placed in high cavities suffer lower predation rates and enjoy higher success for persistence (Joy 2010). In this regard, birds counteract environmental adversity by adapting their behaviour.

Our analysis buttresses the theory of ‘realised ecological niche’ – a nesting site where birds would actually thrive amidst adversaries (Sinclair et al. 2006). However, while there are still some species that have adapted to numerous disturbances in the Mwekera forests, dead wood which provides preferred resources for cavity-nesting birds are more prone to fires and collection for human use than fresh standing trees, especially in unmanaged forests (Stephens 2004). In addition, due to anthropogenic disturbance, juvenile and coppicing trees are often not allowed to grow older to provide decaying portions for creation of cavities. The sourcing of firewood is also selective because local communities target certain tree species, in particular those with hard wood which often provide ideal habitats for cavity-nesting birds.

Avian species conservation in changing environments demands that there should be monitoring, integrated land management and concerted effort (Nelson 2008). Thus, physical and cognitive aspects of conservation need to be addressed to ensure persistence of cavity-nesting birds. Physical aspects of such interventions include implementation of policies, forest management plans and best practices. Cognitive aspects of the appropriate interventions include integrating supportive perceptions and attitudes in resource users. For instance, stakeholders’ awareness of ecological processes and the structure of miombo woodlands associated with bird conservation are critical in the light of many forest provisioning functions and services. However, attempts to implement the Forest Policy of 1998 in Zambia face challenges as the legal aspects of forest resource management are still under the Forest Act of 1973, despite having the Forest Act of 1999 formulated. The old Forest Policy and Forest Act in use do not suit the changing environment, making understanding and appreciation by multiple stakeholders of the role of natural resources difficult.

Anthropogenic activities such as the lighting of fires, collection of dead wood, and land clearing affect the existence of forested lands, thereby threatening the persistence of cavity-nesting birds (Lee & Irwin 2005; Newton 1998). For instance, small patches of woodlands isolated by human activities are avoided by the cavity-nesting birds even if they contain suitable nest sites (Loman 2006). Therefore, steps are needed from multiple stakeholders, including wildlife managers, policy makers and local communities to ensure the preservation of bird diversity and retention of ecological processes such as nesting, breeding, feeding and roosting. Such steps will help local communities derive benefits from ecosystem services such as ecotourism, which depends on birds and other wildlife species (Snyman 2012). Further, for cavity-nesting bird restoration programmes, there is need to explore and investigate effectiveness of use of artificial nest sites (Wilson & Hockey 2013) and rehabilitation of disturbed woodlands (Benayas & Bullock 2012; Gomez 2014).