Birds of a Euphorbia-Acacia Woodland in Ethiopia: Habitat and Seasonal Changes

Abstract

The objective of this study was to understand better the dynamics of a species-rich tropical bird community. This paper reports the analysis of the habitat and the seasonal trends, and a later paper will give a detailed account of the species recorded. Reviews of data on breeding seasons in the tropics have been published by Baker (1939), who discussed general trends, by Skutch (1950) for Central America, Snow & Snow (1964) for northern South America, Moreau (1950, 1966) for Africa generally, Benson (1963) and Winterbottom (1963) for southern Africa, Voous (1950) for Indonesia, Betts (1952) for southern India, Keast (1959) and Frith & Davis (1961) for Australia. These compilations utilize data over large areas. Several detailed accounts of local areas are also relevant to this study, such as those of Davis (1945) in Brazil, Davis (1953) in Guyana, Miller (1963) in Colombia and Snow & Snow (1964) in Trinidad. The possible role of staggered nesting as a factor in species diversity of tropical habitats has been discussed by MacArthur (1964) and by Ricklefs (1966), both of whom concluded that the staggering had little influence on diversity. The study area, 100 x 200 m (2 ha = 4 94 ac), was located in the Main Ethiopian Rift, 9 km south of Lake Langano, and 27 km north-north-east of the town Shashamanne. The substrate is lacustrine sediments and the terrain is very flat. Because it is located on the upper side of a small Tertiary fault line, about 1 km north, the lacustrine layer is much shallower than over most of the Rift floor. The underlying volcanic bedrock averages about 2-3 m below the soil surface. The soil is an alkaline (pH = 7 6-8.2), silty, clay loam, fairly rich in nutrients. The elevation is 1800 m. The climate of this area is not documented, but Fig. 1 shows the monthly rainfall averages for Shashamanne. Although only 27 km away and only 100 m higher in elevation, the town is located on one of the higher points of the Rift floor and is exposed to the moisture-laden south-west winds of the rainy season, whereas the study area is in a rain shadow behind the Shashamanne area. The town is on the border of the Podocarpus gracilior Pilger forest zone, while the study area is in a thorn woodland. The rainfall pattern shown in Fig. 1 is the same as that for my study area, but the average rainfall is undoubtedly less than 50 cm, instead of the 83 cm at Shashamanne. There are early rains in April, usually a short dry spell in late May and early June, and then more extended rains until October.