The Place of Africa in the Palaearctic Migration System

Abstract

The migration of Palaearctic birds southward in autumn and northward in spring has long been recognized as one of the most remarkable biological phenomena. In modern times much attention has been given to the behaviour of the individual species and of the individual bird on migration and also some attention, though relatively very little, to the behaviour and ecology of the migrants in their winter quarters. But there has been no review of the Palaearctic migration as a whole in its essential aspect as a seasonal ecological adjustment on a gigantic scale. In the present paper an attempt will be made to summarize the broad geographical facts which determine the main outlines of the migration system and which render Africa south of the Sahara (the Ethiopian Region) of paramount importance in the survival of many species prominent in the Palaearctic avifauna. The climates which the migrants encounter during their stay in the Ethiopian Region and the habitats into which they distribute themselves will then be briefly described and the problem of competition in their winter-quarters will be discussed. Finally it will be emphasized that, owing to changes in both the Palaearctic and Ethiopian environments, the details of the migration system are always changing; and some extreme differences that can be deduced to have occurred in the past are indicated. For the present purpose information about the distribution of Palaearctic birds in their breeding ranges and in their winter quarters outside the Ethiopian Region is derived from Hartert (1910-38). Data relating to Africa are taken from the compilations of Sclater (1924-30) and Grote (1930, 1931, 1937), supplemented by later, incidental, references and by unpublished observations by myself and others, which it is hoped to collate in detail elsewhere. Trinomials are used only where essential. The extent to which the various Palaearctic species move south in autumn shows extreme variation. Among those which travel an appreciable distance, some, like the linnet (Carduelis cannabina), reach the southern edge of the Palaearctic, as on the south shore of the Mediterranean, but go no farther. Others, like most of the waders (Limicolae) and the grey wagtail (Motacilla cinerea), winter extensively in both the Temperate Zone and the Tropics. Others, again, like the blackcap (Sylvia atricapilla), winter in the southern part of the Palaearctic and in the tropics. Finally, we have the long-distance migrants par excellence, exemplified by the swallow (Hirundo rustica), which leaves the Palaearctic completely, except for stragglers. Table 1 shows approximately, by families, what Palaearctic birds migrate wholly or mainly out of the Palaearctic Region. Certain pelagic groups, e.g. auks and sea-