If there were no warmer lands to the south, the Palearctic avifauna would be considerably poorer. (David Lack 1954.)

Each autumn, after breeding in Eurasia, many millions of birds, from tiny warblers to large eagles, travel to wintering areas in tropical Africa, and back again next spring. This is perhaps the most impressive migration system in the world, not merely because of the huge numbers of participants, but also because of the long and formidable journeys involved. Birds from the west of Eurasia must cross the Mediterranean Sea and Sahara Desert, and those from further east must cross the deserts of southwest Asia and Arabia. Many of the travellers must also negotiate high mountain ranges, such as the Alps in the west or the Himalayas in the east (Figure 24.1). Moreover, once in Africa all these migrants cram into a geographical area less than half the size of their Eurasian breeding grounds, adding to an avifauna already rich in native species (Moreau 1972). Those that winter north of

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Figure 24.1 Map showing the desert and mountain areas to be crossed or circumvented by Palaearctic migrants that spend the northern winter in Africa south of the Sahara.

the equator do so in a season of progressively deteriorating conditions. On their return, many migrants must fatten for the journey in the Sahel zone, just south of the Sahara, at the driest time, when most types of food are near their lowest level of the year. Many European species that winter in Africa have declined in recent decades, raising questions about whether the causal factors lie in the breeding or wintering areas. The distributions, ecology and movements of the migrants within Africa have therefore attracted considerable interest, as have the factors that influence their population levels.

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