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2.2. Methods
Field equipment and materials used during data collection period include binoculars, digital photo camera, GPS, and data sheet. Bird guidebooks (Stevenson & Fanashawe, 2002, Redman et al. , 2009; Ayanlem, 2013) were used for bird identification. Preliminary survey was conducted in May 2016 to gather relevant information about the study area. In this survey, an overall view of the study area was assessed. Global Positioning System readings were used to locate the positions and to identify the altitudinal ranges of the study area.
Based on the habitat type, the study area was classified into swampy, open-land, and forest patch. Sampling units representing each habitat type was selected based on stratified random sampling method. The technique involved dividing the study area into blocks by choosing the location of each habitat with random numbers as adapted from Sutherland (1996).
Point count method was used to record the presence and abundance of bird species (Bibby et al., 2000). Data were taken on 30 sampling points of which 13 sampling points on swampy habitats, 9 on forest patches, and 8 on open habitats. In each point count station, a minimum distance of 150-200m was maintained using GPS to avoid double counting (Vielliard, 2000). Point count method was undertaken from a fixed location within the sample unit of radius of 25m at a fixed time interval consisting of 5-10 minutes depending up on the difficulties of the area and the type of bird species to be identified and enumerated (Vielliard, 2000; Buckland, 2006). To minimize disturbance during counts, a waiting period of 3–5minutes prior to counting were applied (Hostler & Martine, 2001). All birds seen were recorded, except birds flying over the canopy and not stopping within the 25m radius.
Data were recorded in the morning from 06:30–10:00 a.m. and late in the afternoon from 15:00–18.00 p.m. for five consecutive days per month for six months both during the wet and dry seasons. Wet season data were collected during June, July, and August 2016, while data for the dry season were collected during February, March, and April 2017. Identification of avian species was carried out using plumage pattern, size, shape, and color of birds (Aynalem & Bekele 2009). Photographs were taken to confirm identification of some of the avian species which were not easily identified in the field. The taxonomic groups of birds were categorized based on field guides (Stevenson & Fanashawe, 2002, Redman et al. , 2009; Ayanlem, 2013).
2.3. Data analysis
Data for the abundance of birds and the effect of habitat types and season on birds were analyzed using ANOVA, and avian diversity was analyzed using Shannon-Wiener Diversity Index (H’). Simpson′s Index of Diversity (D) was used to evaluate the relative abundance of avian species in each habitat type. Species evenness was assessed using Shannon’s equitability index (E). Relative abundance of avian species was determined using encounter rates (Bibby et al., 1998), and avian community similarity were assessed using Sorensen’s coefficient as adopted from Jeffery et al. (2004). SAS (Version 9.2) software program was used to run the analysis and Tukey’s HSD test at α=0.05 was used to test significant differences.
3. RESULTS
A total of 131 bird species belonging to 15 orders and 53 families were identified of which 122 and 108 species were recorded during the wet and dry seasons, respectively. Among the total avian species, 99 species were recorded both during the wet and dry seasons, 23 species were recorded only during the wet season, and 9 species only during the dry season (Table 1).