To all appearance, far less food is available to wintering migrants north of the equator than south of it. As plant growth occurs in the wet season (northern summer), this is the time of greatest productivity in areas north of the equator, so the majority of migrant species which stay in these regions experience progressively worsening conditions from the time they arrive in Africa to the time they leave. The same is true for wetland birds, for during the rains water collects in every hollow, and rising rivers may flood extensive areas. But as the dry season advances, these productive waters progressively shrink and disappear. Towards the end of the dry season, in the Sudan and Guinea zones, grass fires become widespread. As the grasslands dry at different times in different regions, a 'fire belt' spreads systematically through large parts of the continent each year in more or less regular pattern. Within this belt, the fires are localised at any one time, but over a period of weeks most of the terrain gets burnt, and the belt moves on. Small animals flushed or injured by grassland fires are exploited by a wide range of birds, including raptors, storks, rollers, bee-eaters and others. They include many Palaearctic migrants which are free to follow the fires across the continent, on a roughly similar schedule from year to year. In any one locality, however, the feast is short-lived, and in effect fires destroy many of the remaining food supplies. They affect especially the food available at ground level and, through scorching, also that in the lower trees and bushes. Yet despite the seasonally worsening conditions, many more migrant species somehow maintain themselves in the Sahel zone just south of the Sahara than anywhere further south. Moreover, they are able to fatten there in preparation for the spring desert crossing.
In contrast, those birds that move on south of the equator, where the seasons are reversed, experience progressively improving conditions. The rains are starting as the first migrants arrive in late September and have hardly finished when they leave in March-April. Some 53 landbird species commonly reach the equator, and 31 of these extend to 5-25° south (Pearson & Lack 1992), effectively following the intertropical rain belt and benefiting from humid conditions during most of their stay in Africa. However, these birds must still pass north through the arid Sahel zone on their return migration, getting their last chance to feed and fatten before a long desert-crossing. They do not, therefore, escape the drought conditions entirely, although many fatten in the more mesic Sudan and Guinea zones to the south, where the rains begin in March-April, rapidly increasing local food supplies. In particular, the first rains trigger the swarming of ants and termite alates, a superabundant and energy-rich food source favoured by many species, from passerines to raptors. Many of the migrants that fatten in the Sudan and Guinea zones before setting of on spring migration may then over-fly the Sahel zone (where rain has not yet fallen) along with the Sahara, having little or no chance to feed again until they reach North Africa.
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