The study on species diversity ecological changes of bird habitats and their impact on birds was carried out in Koga reservoir from October 2010 to March 2011. The study area was divided into three habitats types based on the vegetation stratification and the topographic feature of the area. The habitats were wetland, farmland and scrub land. Data collection was conducted using point count and transects count. 103 bird species were recorded during the wet and dry seasons including Palaearctic, intra-African, both African and northern migrants, endemic, globally threatened and vulnerable species. The most predominant families were Accipitridae 11 species followed by Ardeidae with six species. Geese, cranes, doves, starling, pigeons, fire finches and Ibis were the most abundant species. Whereas pallid harrier, marabou stork, Thick billed raven African fish- eagle were amongst the rare species High diversity index was recorded at scrub land (3.5599) and (3.12_60) during both dry and wet seasons whereas the farmland had the lowest diversity index (1.955) during the wet season and wetland (2.691) during the dry season. The highest species composition and richness of birds recorded were 62 species during the wet and 70 species. during the dry season in the wetland. Birds were lived on wetland before dam and fed mainly on wetland. After constructioa of the dam water was stored as a result new birds came towards and the natives increased numbers. Thus those carnivores were ,,.- beneficial,but seed feeders shifted to their feeding habits farmers crops and remained pests. Threats of birds are associated with habitat fragmentation due to anthropogenic and natural. Human population densities and land use, over farming activities, deforestation of modified plants, sedimentation through flooding were factors that affected the breeding sites of the birds . .... ABSTRACT