On the altitudinal limits of birds on BasilC Peak (Bioko Island; Equatorial Guinea)

Abstract

Pérez del Val, J. 2000. On the altitudinal limits of birds on Basile Peak (Bioko Island; Equatorial Guinea). Ostrich 71(1 & 2): 342.

The lower altitudinal limit of the montane avifauna in Africa has generally been set at 1 500- 800 m, corresponding to vegetation belts of the main mountain ranges of East Africa. However, much lower limits have been reported from the East Usambaras (Tanzania), Mount Cameroon, and Bioko Island. Basilé Peak is a volcano of 3 011 m which constitutes the northern half of Bioko Island. Below 400–900 m the forest has been logged and largely replaced by cocoa plantations, but the vegetation is relatively undisturbed at higher altitudes. Annual rainfall ranges between 2 000 mm and 3 500 mm. From January 1990 to July 1992 mist-nets were set at 13 standardised stations on Basilé Peak; 1 427 birds of 71 species were captured. Previous data from non-standardised mist-netting at 6 other localities, together with sight and sound records, published data, and 1 047 skins in the collections of Museo Naçional de Ciençias Naturales (Madrid), Estacion Biologica de Donaiia (Sevilla) and the Natural History Museum (Tring) were also used to compile distribution lists. From published botanical surveys, I recognised four vegetation zones: Lowland Rain Forest up to 800 m; Mountain Rain Forest at 80CL1 800 m; Araliaceus Forest 1 800–2 500 m; and Ericaceus Stands above 2 500 m. An analysis of similarity (using the index of Czechanowski) grouped 68 bird species into altitudinal classes at 200 m intervals. This suggests a division between lowland and montane birds at 800–1 000 m, broadly coinciding with the change between Lowland and Mountain Rain Forest. There does not seem to be a discrete bird community restricted to either of the upper zones (Araliaceus Forest or Ericaeus Stands), with only the Yellow Bishop Euplectes capensis restricted to the upper 500 m of the mountain. Most species which reach the summit are found in all vegetation zones, notably the endemic subspecies of the African Goshawk Accipiter tachiro lopezi. It is suggested that low temperatures, high rainfall, and in particular the seasonal distribution of rainfall, may explain the lower altitudinal limits of the montane avifauna on Bioko and on Mount Cameroon.