The forest covers an area of 18,300 ha Forest Reserve with an altitude of 1,550 - 1,650 meters. It was first gazetted as Trust Forest in 1933, and two small nature reserves, Yala and Isecheno (700 ha), were established within the Forest Reserve in 1967. In 1986, nearly 4,000 ha of the northern portion of the forest, along with the adjacent 457 ha Kisere forest, were gazetted as a National Reserve, managed by the Kenya Wildlife Service. The site is a mid-altitude tropical rainforest, the easternmost outlier of the Congo Basin forests. Its West African affinities are unique in Kenya, and the forest contains many species found nowhere else in the country.

The forest lies in the Lake Victoria catchment, 40 kilometers north of Kisumu, and just east of the Nandi Escarpment that forms the edge of the central highlands. Only an estimated 10,000 hactares of the overall gazetted area is still closedcanopy indigenous forest, of which some 3,200 hactares is in the National Reserve. The remaining area consists of grassy and bushed glades (some natural, some maintained by fire or grazing), tea, cultivation and 1,700 ha of plantations of softwoods and commercially valuable hardwoods.

Kakamega forest is an important water catchment; the Isiukhu and Yala Rivers flow through the forest and gather tributaries from it. The terrain is undulating, with often steep-sided river valleys. The soils are well-drained, deep, heavily leached, clay-loams and clays, of generally low fertility. Rainfall is c.2, 000 milimeters per year, decreasing from south to north, and apparently declining due to deforestation. The avifauna is well known, rich, and unusual in its composition. Two globally threatened species occur, Muscicapa lendu (scarce resident) and Eremomela turneri (reasonably common). The 194 forest-dependent bird species (the highest total for any Kenyan forest) include many of Kenya's Guinea - Congo Forests biome species, as well as 33 of Kenya' s 70 Afrotropical Highlands biome species. The mixture reflects Kakamega's altitudinal position between lowland and montane forest.

Kakamega's avifauna is unique not only nationally, but continentally. Several species have isolated, relict populations here, including Andropadus ansorgei, Merops muelleri, Muscicapa lendu and Eremomela turneri, which are absent from all or nearly all of the superficially similar mid-elevation forests in Uganda. Muscicapa lendu is a restricted-range species that characterizes the Kakamega and Nandi forests Secondary Area, and is also present in the Albertine Rift mountains Endemic Bird Area. The presence of the eremomela indicates biogeographic links to the Eastern Democratic Republic of Congo lowlands Endemic Bird Area. Kakamega itself has few endemic taxa; among birds, there is an endemic subspecies (kavirondensis) of Andropadus ansorgei.

At least 16 bird species occur in Kakamega but nowhere else in Kenya, and another 30 (such as Psittacus erithacus) are probably now confined to this site. The grassy glades have their own distinctive avifauna, with many moist-grassland species that are now rare elsewhere in western Kenya. Regionally threatened species include Circaetus cinerascens (fairly common resident), Hieraaetus ayresii (relatively abundant), Stephanoaetus coronatus (resident in small numbers), Tyto capensis (no recent records), Glaucidium tephronotum (widespread at low density), Indicator exilis (not uncommon), Indicator conirostris (uncommon), Prodotiscus insignis (rare, with few recent records), Phyllastrephus baumanni (not uncommon, but rarely recorded), Kakamega poliothorax (extremely local and generally scarce), Sheppardia polioptera (patchily distributed), Hyliota australis (uncommon in forest canopy), Dyaphorophyia concreta (very local), Campephaga quiscalina (rarely recorded) and Euplectes hartlaubi (local).

Kakamega Forest has 410 bird species of which 194 are forest-dependent, two globally threatened species are known to occur in Kakamega, Chapin's Flycatcher and Turner's Eremomela, others bird species includes; Pygmy Kingfisher, Yellow-crested Woodpecker, Black-billed Turaco, Martial Eagle, Crowned Eagle, Black and White Casqued Hornbill, Jameson's Wattle-eye, Red-breasted Owlet, Shining-blue Kingfisher, Blue-breasted Bee-eater, Chubb's Cisticola, Yellow-billed Woodpecker