Patterns and correlates of bird migrations in sub-saharan Africa

The common mode of migration among Afrotropical birds is movement from trop ical to temperate areas to breed usually coinciding with the onset of summe r and the rainy season. The proportion of species that are migratory can be predicted with considerable accuracy from the average temperature of the c oldest month of the year. Once this exceeds 20 degreesC, 90% or more of bre eding species will be non-migratory. The likelihood of any one species bein g migratory is strongly influenced by diet, foraging mode as influenced by the behaviour of prey and vegetation geography. Insectivores are disproport ionately 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 exclus ively hunt aerial insects above the canopy (e.g. swallows, swifts and night jars), perch-hunters that depend on large, active insect prey (e.g. halcyon id kingfishers and rollers) and taxa heavily dependent on the larvae of fly ing 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 a ttributes and hunting behaviour to migratory behaviour, is probably mediate d by a parallel gradient in seasonal prey availability. In marked contrast to the Neotropics and the Orient, many Afrotropical birds undertake polaris ed 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 ar e not replicated in either the Neotropics or the Orient.