The Place of Africa in the Palaearctic Migration System

Abstract

Migrants that leave the Palaearctic Region in winter are in the main the most insectivorous spp. In Britain nearly one-third of the breeding passerine birds depend for winter-quarters on Africa south of the Sahara. The Western Palaearctic has the potential wintering ground provided by tropical Africa, twice its own size, due south of it. On the other hand, the potential wintering grounds south of the Central and the Far Eastern Palaearctic are far smaller than these are. Consequently much of Palaearctic migration is directed south-west in autumn; Africa is sought by nearly as large a proportion of birds breeding in the Central as in the Western Palaearctic and even by some Far Eastern birds. Some of the migrant spp. reaching Africa winter only north of the equator, some south, some throughout. Most spp. populations breeding in different areas are not segregated in winter. Between the Sahara and the equator migrants arrive after the annual rains and experience increasing desiccation throughout their stay. Although few of the resident birds are breeding at that season it is surprising that so many immigrants can be supported under these severe conditions. South of the equator the migrants arrive in time for the annual flush of vegetation, when the local birds are beginning to breed. Hence both resident and immigrant populations reach their peak together, at the time of apparent maximum of food. In some localities and in some genera the immigrant birds outnumber the residents. Details of ecological adjustment in winter-quarters need study. Many immigrant birds appear to share a (possibly superabundant) food supply with allied resident spp.; some are highly itinerant. At least one sp. finds an unoccupied niche. Africa has probably been an important wintering area since early in the Tertiary. The numbers involved varied most during the Pleistocene, with only one-third as many birds at a glacial maximum as during a climatic optimum. The post-glacial changes in the migrant flood have also been great, owing to the effects of human activity.