Patterns and Correlates of Bird Migrations in Sub-Saharan Africa

Abstract

The common mode of migration among Afrotropical birds is movement from tropical to temperate areas to breed usually coinciding with the onset of summer and the rainy season. The proportion of species that are migratory can be predicted with considerable accuracy from the average temperature of the coldest month of the year. Once this exceeds 20˚C, 90% or more of breeding species will be non-migratory. The likelihood of any one species being migratory is strongly influenced by diet, foraging mode as influenced by the behaviour of prey and vegetation geography. Insectivores are disproportionately well represented among the migrants whereas, at the other extreme, frugivores are almost exclusively sedentary. Within the insectivore guild, the greatest migratory tendencies are found among those groups that exclusively hunt aerial insects above the canopy (e.g. swallows, swifts and night-jars), perch-hunters that depend on large, active insect prey (e.g. halcyonid kingfishers and rollers) and taxa heavily dependent on the larvae of flying insects (cuckoos). Insectivores that hunt or glean small invertebrates (volant or not) within the canopy or glean relatively sessile prey from the ground are much less likely to be migratory. This gradient, linking prey attributes and hunting behaviour to migratory behaviour, is probably mediated by a parallel gradient in seasonal prey availability. In marked contrast to the Neo-tropics and the Orient, many Afrotropical birds undertake polarised migrations, with part of the population moving north, and part south, of the tropics. The explanation of this is hypothesised to lie in the spatial symmetry and large extent of savannas both north and south of the tropics, coupled with a lack of north-south dispersal barriers. These conditions are not replicated in either the Neotropics or the Orient.