Long-term dynamics of a tropical savanna bird community

Abstract

Complete two-month censuses of diurnal raptors and strip transect counts of other bird species were compared between 1968–1972 and 1996 to assess changes in the bird community of a 2700ha guinean savanna in Ivory Coast. Among 117 non-raptor species recorded at least once on the transects, 25 mostly uncommon species were absent in 1996 (but only two of them were likely to be extinct in the study area), 4 were first seen in 1996, 14 decreased and 4 increased significantly. All six large forest raptors and resident savanna eagles disappeared before the mid-1980s. Among the 12 remaining breeding raptors, 4 species increased (including the 2 seasonal migrants), 4 species remained stable (including the 2 forest accipiters) and 4 species decreased, but not significantly (including the 2 largest taxa). All 14 non-breeding raptors (Palearctic and African migrants) remained stable or increased. With on average 63 breeding pairs per 1000ha, this area still supports one of the highest densities of diurnal raptors ever recorded. The geographic distributions, seasonal movements and ecological requirements of species involved suggest the most likely origin of specific changes observed. They were attributed, in roughly similar proportion, to: (i) decreasing rainfall (southward shifts leading to appearance or increase of some resident or migrant species); (ii) deforestation, agricultural development and hunting pressure around the reserve (extinction of larger species following habitat fragmentation or degradation, increase of some species); and (iii) habitat modifications inside the reserve (increasing tree cover, invasion of dense thickets, illegal exploitation of palms). The overall community of this small protected area, however, exhibited a remarkable stability during the last 30 years, in spite of dramatic habitat changes outside its limits.