Bird Diversity in Nensebo Moist Afromontane Forest Fragment, South Eastern Ethiopia

We employed a stratified random sampling technique with our study area stratified into two dominant habitat types: moist Afromontane forest and modified habitat. Within strata, we established 20 transect lines of 1km length and 0.25km width to sample 27.75% of the study area. We used line transect count methods aided by binoculars to estimate avian species diversity and distribution. We employed quantitative biodiversity indices, such as Shannon wiener diversity indexes to compare species diversity among habitat types and two way ANOVA to analyze the effect of season and habitat on bird species richness and abundance.


INTRODUCTION
Due to easy detectability and ease of study, birds are wellknown species to the scientific community [1]. However, despite the fact that birds are well studied, new species are periodically being discovered in relatively unexplored regions of tropical and sub-tropical rainforests [2,3]. Forests are known to be home to about 75% of avian species (>5,000 species) [4]. Within these habitats, avian species provide a wide range of ecosystem functions and services, such as pollination and seed dispersal. For example, over 920 species of birds pollinate plants [5], while within some tropical forests, up to 90% of the tree species are dispersed by animals, mainly mammals and birds [6]. However, the extinction of forest birds, mainly caused by forest loss, fragmentation and degradation, has been escalating over the last few decades [7]. Particularly, about 86% of globally threatened bird species are known to be at risk due to deforestation [8]. As a result, there is a need to document bird species diversity over unexplored remote regions of the world.
While several studies have focused on characterizing bird species composition and abundance of specific regions around the world [9 -13], documentation of global avian diversity is still incomplete. Particularly, in Ethiopia, much of the remote areas remain unexplored in terms of avian diversity, thus, making Ethiopian avian checklists far from complete. Ethiopia is known for its rich bird diversity [14]. According to Birdlife International (2018) [8] Ethiopia is home for 819 species of birds, out of which, 35 are globally threatened and 17 are endemic to the country. Additionally, ~237 Palearctic and inter African migratory bird species have been recorded from Ethiopia (BLI 2018) [8]. Ethiopia has 68 important bird areas (10,794,408 ha) and 4 endemic bird areas (BLI 2018) [8].
Various studies in Ethiopia have characterized bird species diversity in protected areas [10, 15 -17]. However, reports of bird species diversity outside protected areas in Ethiopia are very limited [18,19]. About 17.62% of Ethiopia land mass coverage is protected (United Nations Environmental Protection-World Conservation Monitoring Center (UNEP-WCMC) 2019) [20]. However, a large proportion of important bird areas of Ethiopia lie outside protected areas, where information on bird species diversity is sparse. Most of these areas are also highly fragmented, located in human dominated landscapes that are often threatened by anthropogenic impacts. As a result, there is a need to document bird species diversity outside protected areas as a step towards the completion of avian records of Ethiopia, as well as provide important information for sustainable conservation of bird diversity and their habitats within this region.
Nensebo forest, one of the important bird areas of Ethiopia, is a patch of moist Afromontane forest partly connected to the Bale Mountains National Park (BMNP). However, with continued human encroachment, the wildlife corridor between the Nensebo forest and the BMNP has been reduced. Despite the importance of this region, there has been no scientifically documented information on bird species composition and abundance for this region, despite the need of this information for management actions and sustainable conservation for this region. Therefore, this study was implemented to investigate species diversity, abundance and distribution of birds at Nensebo forest.
Nensebo woreda is characterized by a mountainous landscape having an altitude range between 1500 m a.s.l to 3700 m a.s.l. The Nensebo woreda exhibits bimodal rainfall pattern, with the annual rainfall range between 900 to 1100 mm and with a temperature that varies between a minimum of 15 °C and a maximum 22 °C (NWAO 2012) [21].
Nensebo forest is a moist Afromontane forest jointly managed by the community and Oromia Forest and wildlife Enterprise. Nensebo forest is part of Bale Mountains Ecoregion known for its rich biodiversity and high level of endemism. Nensebo forest is one of the remnant moist Afromontane forests in the southeastern part of Ethiopia that exists in human dominated landscape [22]. The total area of Nensebo forest is 11,350 ha, comprised of both relatively intact moist Afromontane forest and modified Afromontane forest.
The moist Afromontane forest was characterized by dominant stands of the indigenous tree species such as Croton macrostachys, Strychnos spinosa, Clematis longicauda, Prunus africana and Millettia ferruginea [22]. The forest occurred over mountain slopes, valleys and remote inaccessible areas. The level of disturbance was minimal, there were no settlements or cultivation, and hardly any livestock grazed in the area. Modified moist Afromontane forest (habitat) occurs relatively at lower altitudes (1882-2153 m asl) in close proximity to human settlement and cultivation areas. Modified habitat is characterized by sparse stands of moist Afromontane characteristic tree species with mild crop cultivation (mainly 'enset' and coffee), intense livestock grazing and sparse human settlements [23]. The modified habitat is a heterogeneous habitat that provides diverse feeding guilds including fruiting tree crops such as coffee, as well as cereal crops such as barley and wheat. At the present, the modified habitat harbors perching trees Croton macrostachys and Millettia ferruginea and shrubs that provide good nesting grounds [22].

Sampling Design
We used line-transect methods to document the avian community due to better probability of detection, relatively easy accessibility of the study area and to cover a large sample area [24]. Based on vegetation types and altitudinal range, we first stratified the study area into two dominant habitat types; namely moist Afromontane forest and modified habitat. We then systematically generated sampling transects in a geographic information system (GIS) using ArcGIS software v. 10.1 (ESRI 2012) [25] representing the two dominant habitat types. We employed a stratified sampling technique in which transect placement was proportional to the area of the habitat types and represents each of the habitat types [15,24]. Accordingly, we established a total of 20 transect lines, of which six transects were in modified habitat while the remaining fifteen (15) transects were in moist Afromontane forest (Fig. 2). The distance between two adjacent transects was 1 km and the length of each transect line was 1km with a maximum width of 0.25km. To avoid edge effects, we spaced transect lines 0.5km from the roadside (edge of the forest). Over the two dominant habitat types, established transects covered ~ 27.75% (3150 ha) of the study area.

Data Collection
We carried out data collection from August 2017 to January 2018, covering both wet and dry seasons. We conducted ground monitoring (walking on foot along transect lines) to survey bird species diversity in the study area. We recorded all birds (number of individuals and species) seen (with the naked eye and aided by binoculars (10x500)) while traversing transect lines. For further confirmation of species identification, we used digital sound recorders to record bird sounds, as well as digital photo cameras to take pictures of the birds. We counted birds when they were active during early morning hours (06:30 -10:00 AM) and late afternoon hours (from 15:30 -18.00 PM). We avoided bird counting during unfavorable weather (strong wind, cold or rain) conditions. We identified bird species using key taxonomic and behavior features, such as plumage pattern, size, shape, color, songs and calls [26] with the aid of guide book Redman et al. [27].

Data Analysis
We computed Shannon-Wiener diversity index (H') and evenness [28] for each habitat type to compare diversity among the two dominant habitat types. We calculated Shannon-Wiener diversity index using the formula: Shannon's index (H')' = ∑ (pi) (lnpi), where, i= is the proportion of the species relative to the total number of species (pi) multiplied by the natural logarithm of this proportion (lnpi) and the final product multiplied by -1. Similarly, we calculated species evenness by the formula: Evenness (E) = H'/ Hmax, where, Hmax is defined as ln (S) and species richness (S) is defined by: S = ∑ n, where n is the number of species in the community. We calculated relative abundance of bird species using the formula (%) = n/N × 100, where n is the number of individuals of particular species recorded and N is the total number of individuals of the species. We then used a two-way analysis of variance (ANOVA) to analyze the effect of season and habitat on bird species richness and abundance.

Species Composition and Relative Abundance
A total of 105 species were recorded from the Nensebo forest ( Table 1). Out of the recorded species, 41 and 8 species exclusively occurred in the moist Afromontane and modified habitats, respectively, whereas 56 species were present in both habitat types. Among the recorded species, 8 near endemic (endemic to Ethiopia and Eritrea) species, namely Wattled ibis (Bostrychia carunculata), Thick-billed raven (Corvus crassirostris), Black-winged love bird (Agapornis taranta), Ethiopian cisticola (Cisticola lugubris), Abyssinian oriole (Oriolus monacha), Abyssinian slaty flycatcher (Chocolatina ficedula), Abyssinian woodpecker (Abyssinicus phylloscopus) and Banded barbet (Lybius undatus) were recorded. Out of the near endemic species recorded, Ethiopian cisticola occurred exclusively in the moist Afromontane forest, whereas other species were present in both habitat types. The endemic Yellow-fronted parrot (Poicephalus flavifrons) exclusively occurred in the moist Afromontane forest habitat. Furthermore, the globally threatened Semi collared flycatcher (Semi torquata) was confined in the moist Afromontane forest habitat ( Table 1).
Within the Nensebo forest, Sylviidae (10 species) was the most dominant family followed by Accipitridae (8 species), Columbidae (7 species) and Cisticolidae (6 species) ( Table 1). Among the recorded bird species, 96 birds were resident whereas 6 were Palearctic migrant species and 3 were inter-African migrants ( Table 1). Inter-African migrants were confined in moist Afromontane forest, whereas most of the Palearctic migrants occurred in the modified habitat. Higher species richness and abundance were recorded during the wet season than the dry season in both habitat types ( Table 1).

DISCUSSION
The bird diversity study at Nensebo forest revealed that this area supports high diversity of birds, including several endangered, endemic, and migratory species. The diverse plant assemblages and presence of modified habitats such as coffee based and 'enset' based agroforestry systems around the forest [22], could support rich bird diversity. The structure and complexity of habitat are the two main factors influencing avian community diversity [29,30]. Vegetation diversity and structure influence microhabitat conditions, such as feeding guilds that can be used as predictors of bird species richness and abundance in forest ecosystems [30,31]. The variations in abundance of bird species among seasons might also be related to the availability of food, habitat conditions and breeding season of the species [10, 32 -34]. Plant species diversity and structure are influenced by complex geographical and environmental gradients, such as rainfall pattern, temperature, altitude, aspect [9,29]. As a result, the rainfall variation between wet and dry seasons could create variations in vegetation diversity and abundance that ultimately lead to variations in bird species richness and abundance. During the wet season, flowering plants flourished and as a result, food was plentiful for birds in almost all the habitats [22]. However, during the dry season, the deciduous trees defoliated and in the absence of food, many species of birds were restricted to specific habitat where sufficient resource was available. Furthermore, during the wet season, the surrounding agricultural lands are covered by crops that provide alternative temporary seasonal foraging and nesting opportunities to the birds [23]. This could decrease their abundance in their natural habitat during the wet season, particularly in the modified habitat type [15,34,35].
The higher diversity and evenness of bird species in the modified habitat than the moist Afromontane forest habitat could be due to variations in habitat heterogeneity. Modified habitat had more heterogeneous vegetation and vegetation strata as compared to moist Afromontane forest habitat that could provide several niches for birds promoting high bird species diversity per given area sampled. Particularly, cultivated fruiting trees and shrubs such as coffee and cultivated crops provide better foraging opportunities particularly during fruiting season, whereby high concentrations of birds were observed. Similar studies elsewhere in the world unanimously state that habitat heterogeneity promotes bird species diversity through providing increased feeding guilds and nesting op-portunities [10, 16, 26, 32 -36].
However, considering the overall species richness, the unmodified moist Afromontane forest harboured the highest richness due to its large size, better cover opportunity and little disturbance. Moreover, the habitat is home to 41 forest specialist bird species such as the yellow-fronted parrot, African paradise flycatcher (Terpsiphone viridis), Abyssinian oriole, black-headed oriole (Oriolus larvatus) and Ethiopian cisticola that exclusively occurred in this habitat type. For example, the Abyssinian oriole had the highest percent relative abundance in the moist Afromontane forest. It has been reported that Abyssinian oriole inhabits highland forests between 900 and 2000 m asl, where it optimally attains its favorite berries, fruits and caterpillars [37]. Similar observations have been made in a study carried out in Wondo Genet forest in Ethiopia [35] and Araninge, Kwamsisi, Gendagenda and Msubugwe forests in Tanzania [38], whereby forest specialists were only confined to the homogenous forest ignoring the heterogeneous habitats. As a result, the homogenous unmodified moist Afromontane forest is critical habitat for the survival of diverse forest specialist bird species.

CONCLUSION AND RECOMMENDATIONS
The Nensebo forest had high bird species diversity including endemic and endangered species revealing the importance of the site for bird conservation. Therefore, it can serve as good potential for bird watching tourism that can integrate economic gain with biodiversity conservation. This study has demonstrated that habitat heterogeneity favors bird species diversity. However, high concentration of bird species around cultivation and settlement areas can bring birds and human in close contact initiating human wildlife conflict. Birds could cause crop loss (economic loss) and in turn, retaliatory kills could happen. At present, the study revealed that the unmodified moist Afromontane forest is critical habitat for diverse forest specialist bird species. Hence, it is important to promote sustainable forest conservation and promote land use planning. Cultivation at close proximity of forest should be avoided and community awareness campaigns should be initiated to aware the local community about the importance of wildlife conservation in general and birds in particular.

ETHICS APPROVAL AND CONSENT TO PARTICIPATE
Not applicable.

HUMAN AND ANIMAL RIGHTS
Not applicable.

CONSENT FOR PUBLICATION
Not applicable.

AVAILABILITY OF DATA AND MATERIALS
The data sets analyzed during the current study are available from the corresponding author (Z.G) upon request.

This study was funded by Hawassa University First Round
Thematic Research year 2016-17.