A Critical Analysis of the Distribution of Birds in a Tropical African Area

Abstract

An analysis of the distribution of 294 spp. of land birds in Usambara shows a sharp horizontal separation between the forest fauna and that of other habitats. Within the forest, the same species were found in communities of highly dissimilar aspect and specific floral composition. The fauna exhibits abrupt vertical zonation into Lowland, up to 2500 ft., Intermediate, 2500 to 4500 ft., and Highland areas, from 4500 ft. upwards. This is marked both within the forest, which ranges from sea-level to 7500 ft., and without. In analyzing the factors effective in limiting distribution, biotic factors involving food, nesting sites and materials, enemies, and interspecific competition are considered of little importance except for a small minority of species. Temp. variables are apparently not sufficiently extreme to act as a general factor in controlling distribution, unless the effect of temp. is exerted upon breeding activities rather than upon physical tolerance, in which case small local differences in temp. may be important. Subjective factors, as the psychological preference for a certain habitat through long evolutionary association with it, may explain the restriction of some species to one of 2 or more similar areas where there are no obvious other differences of significance. The lack of certain knowledge as to the controlling influences in the distribution of birds is emphasized.