North of the equator, the generally dry environments are alleviated by extensive shallow wetlands which occur patchily from west to east, mainly around the Senegal River (in Senegal), the upper Niger River (in Mali), the Lake Chad basin (in Chad), and the Nile-Sudd swamps (in southern Sudan). These wetlands provide productive feeding areas year-round, even at the height of the dry season, leaving rich grassland and pools as they retreat. The seasonally inundated plains around Lake Rukwa, the Lake Victoria basin and the Wembere swamps in Tanzania also support large numbers of birds, including Palaearctic migrants. Each of these areas extends in the wet season over many thousands of square kilometres.

Important wetlands south of the equator include the Bangweulu swamps and the Kafue floodplain in Zambia and the Okavango Delta swamps in Botswana. Each of these areas again extends in the wet season over many thousands of square kilometres. Together with the northern areas, they comprise some of the largest expanses of freshwater wetland on earth, covering an estimated 657 000 km2 of West Africa, 208 000 km2 of East and northern Africa and 172 000 km2 of southern Africa. However, most of these areas are being rapidly reduced by drainage and other human impact, and now cover only a small part of the ground they occupied in the 1970s.

Many observers have commented on the huge numbers of herons, ducks and waders that concentrate in the Sahel wetlands during the northern winter. In West Africa, the greatest numbers occur in the basins of the Senegal and Niger. These two rivers are subject to a single spectacular flood each year, beginning in July, which transforms huge areas of desiccated earth into rich marshland (Morel 1973). Some of the vegetation brought on by the start of the rains has time to produce seeds before it is flooded, providing a food supply for those ducks and waders that eat seeds. In the swamps, true marsh vegetation develops, producing an additional food supply of seeds and aquatic invertebrates. Fishes reproduce in the wet season, and the flooded areas are rich in small fry. As the flood retreats, it leaves behind a great network of shallow pools and channels in which many fish become trapped. It has been calculated that, in the Senegal Delta of about 146 000 ha, at least 30 000 tons of fish are eaten annually by cormorants, herons, pelicans and other waterbirds (Morel 1973). Although the smaller pools dry up by late December, the larger expanses of flood water last longer and afford suitable habitat for waterbirds throughout the winter.

In the Senegal Delta alone, more than 300 000 Pintail Anas acuta, Garganey Anas querquedula and Shoveller A. clypeata have been counted in good years, along with several hundred thousand Ruffs Philomachus pugnax, tens of thousands of Black-tailed Godwits Limosa limosa, and vast numbers of Tringa sandpipers. However, numbers vary greatly from year to year, depending on wetland conditions, as shown for some ducks in Table 24.2. In comparison, even in good years relatively small numbers of African species occur here, mainly from different genera. Among herons, however, the resident and migrant populations of four species occur together, namely Purple Herons Ardea purpurea, Night Herons Nycticorax nycticorax, Squacco Herons Ardeola ralloides and Little Egrets Egretta garzetta, so it is impossible to assess the relative proportions of Eurasian and African birds. These wetland areas also support many other birds, both passerines and non-passerines. For example, the Upper Niger floodlands cover 40 000 km2, and in the dry season support wintering raptors at 12-15 times the densities found in adjacent drier land (Thiollay 1989).

In addition to the inland wetlands, several coastal areas are important. In particular, the Banc D'Anguin off the coast of Mauritania holds very large numbers of shorebirds and waterfowl throughout the northern winter, and is also an

Table 24.2 January counts of the most numerous species of ducks wintering in the Senegal River delta, West Africa, showing the enormous variation between years, 1972-1996
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