Some 186 species of landbirds and 29 seabirds (215 species in all) that breed in Eurasia spend the winter in Africa, south of Sahara. In late summer, they total an estimated 5000 million individuals. These birds derive mainly from the western third of Eurasia, but some also from further east, as far as northwest North America. They all face long and formidable journeys, with long desert-crossings. In the northern winter, they are estimated to form only 6-7% of the total African bird population, but in certain northern savannah habitats they outnumber African species.
Within Africa south of the Sahara, the numbers of migratory species decline southwards, and the greatest diversity occurs in the northeast, coinciding with a region also rich in native African species. Most migrants occur in seasonal woodland, savannah and scrub habitats, which also hold 67% of all African species; however, the migrants have greater mobility, travelling long distances within their stay, and some are nomadic within their wintering range, concentrating temporarily wherever food is abundant (like some intra-African migrants). The Eurasian migrants also differ ecologically from closely allied native species, with at least slight differences in habitats and feeding behaviour.
Many species winter in dry season conditions in the northern tropics which deteriorate during their stay, while smaller numbers move south of the equator to spend the winter in wet-season conditions. Some Eurasian-Afrotropical migrants show single-stage migration to the northern or southern tropics, while others show two-stage migration, either within the northern tropics or between the northern and southern tropics. In consequence, the bulk of the migrant population shifts southward within Africa during the northern winter.
Recent declines in the numbers of some species, detected on European breeding areas, have coincided with reduced rainfall in the northern tropics, especially in the western Sahel zone. In some species, precise relationships have emerged between annual breeding numbers or survival and annual rainfall in African wintering areas. In other species of Eurasian-Afrotropical migrants, long-term declines have been associated with habitat destruction or deterioration in breeding areas. Hunting in the Mediterranean area removes many migrants in autumn, but with no known affects on population trends.
The majority of east Eurasian migrants winter in Southeast Asia (including the Indian subcontinent), with a small number of species (mainly shorebirds) migrating as far as Australia and New Zealand.
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