Distributions

The first point to be made is that the immigrant species do not by any means distribute themselves evenly over the continent (Figure 24.3; Newton 1995). As mentioned above, most species occur in the belts of Sahel and Sudan savannah habitats which lie south of the Sahara from west to east across the continent and which, some weeks earlier, were vacated by the intra-African migrants returning southwards. Within these belts, maximum species numbers occur in the east. From the Sudan zone southwards, the numbers of Eurasian migratory species progressively decline so that few reach the southern tip of the continent. It is as though the birds migrate no further in sub-Saharan Africa than is necessary to find suitable conditions. However, it remains conjectural whether this southward decline in species numbers is due to Palaearctic species simply petering out because their populations can be accommodated further north, or whether adverse ecological factors or competition with Afrotropical species curtail their wintering distributions southwards.

In general, in non-forest habitats, regions with high numbers of Palaearctic migrants also tend to support high numbers of Afrotropical species, as in parts of East Africa (Newton 1995a). This may be partly due to the higher topographical diversity in East Africa, and to the wider range of habitats available there,

Figure 24.3 Numbers of Palaearctic migrant species found in different parts of sub-Saharan Africa during October-March. From Newton (1995b). In addition, a few migratory species occur around oases in the Sahara in winter, notably Eurasian Chiffchaff Phylloscopus collybita, Sardinian Warbler Sylvia melanocephala, Olivaceous Warbler Hippolais pallida, Northern Shrike Lanius excubitor and various desert species (Gaston 1970). These numbers could form at most only a trivial fraction of the total wintering populations of these species. The same is true for the various Palaearctic species that winter in small numbers in some parts of the Arabian peninsula (S. F. Newton 1996).

Figure 24.3 Numbers of Palaearctic migrant species found in different parts of sub-Saharan Africa during October-March. From Newton (1995b). In addition, a few migratory species occur around oases in the Sahara in winter, notably Eurasian Chiffchaff Phylloscopus collybita, Sardinian Warbler Sylvia melanocephala, Olivaceous Warbler Hippolais pallida, Northern Shrike Lanius excubitor and various desert species (Gaston 1970). These numbers could form at most only a trivial fraction of the total wintering populations of these species. The same is true for the various Palaearctic species that winter in small numbers in some parts of the Arabian peninsula (S. F. Newton 1996).

coupled with the dual wet season. Hence, at this broad distributional scale within vegetation zones, the Eurasian species do not concentrate in regions with the smallest numbers of African species. Similar correlations are also apparent at smaller spatial scales, as found for small insectivores in savannah habitats of southeast Kenya and Nigeria where the numbers of both African and Eurasian birds were correlated with the density of vegetation, and hence with insect food supplies (Lack 1987, Jones et al. 1996).

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