Figure 13.6 The greater land areas in the northern than in the southern hemisphere, shown by land area (a) and habitats (b). The habitats refer to vegetation zones before human impact, only parts of which still remain. From Rosenzweig (1992).
Another factor is temperature, which has a steeper downward gradient north of the equator than south of it. For example, in the New World the mean midwinter (January) temperature at the Tropic of Cancer is about 13°C, while at 50°N it is — 15°C, a 28°C difference. In contrast, the mean midwinter (July) temperature at the Tropic of Capricorn is 16°C, while at 50°S it is 0°C, a 16°C difference. Similar hemisphere differences are apparent in much of the Old World too. This difference in steepness of temperature gradient between hemispheres may explain why greater proportions of species leave from temperate latitudes in the northern hemisphere than in the southern. For example, about 29% of species leave completely for the winter from Morocco, but only 6% of species leave from equivalent latitudes around the Cape in South Africa (Newton & Dale 1996, Harrison et al. 1997). It is presumably largely for both these land-related and temperature-related reasons that landbird migration is much more marked in the northern than in the southern hemisphere.
While many landbird species that breed in the northern hemisphere migrate south of the tropics, no landbird species that breeds in the southern hemisphere moves north of the tropics. This difference in long-distance migration might also be attributed to the difference in available land areas between the two hemispheres. Birds migrating south from the northern continents encounter progressively smaller habitable land areas, which could force some individuals to extend far to the south. In general, the numbers of migrants from the northern continents decline with increasing distance southwards into the southern continents, as more and more species settle for the winter. In contrast, birds migrating northward from the southern parts of the southern continents encounter widening land areas, so may need to move less far north before they find sufficient wintering habitat. In Africa, no breeding landbird species migrates north of the Sahara, in South America none (apart from stragglers) penetrate north of Panama, and in Australasia none migrate beyond Lombok in Indonesia (Wallace's Line) in winter. It is not that birds which breed in the southern hemisphere do not move around, but rather that most migration is short distance and partial (involving only part of a population), altitudinal (and also short-distance) or nomadic (as birds concentrate locally in line with sporadic rainfall patterns). Relatively few species make regular long-distance moves. It remains to be discovered to what extent the two sets of migrants, from northern and southern regions, occupy the same niches in the tropics but at different times of year.
Also contributing to the reduced numbers of long-distance migrants among the breeding birds of the southern hemisphere is the virtual lack of tundra-nesting shorebirds, which form a large proportion of the long-distance migrants from the northern hemisphere (although one species, the Red-chested Dotterel Charadrius modestus, occurs on the tundra of the Falkland Islands). Most species of tundranesting shorebirds seen on southern hemisphere shorelines year-round are migrants from the northern hemisphere which do not return north in their first year of life (Chapter 15).
In extent of seasonal migration, pelagic seabirds provide a telling contrast with landbirds. The reduced land areas in the southern hemisphere mean that the sea areas are correspondingly larger there than in the northern hemisphere. Linked with these greater sea areas and large numbers of scattered island breeding sites, pelagic seabirds are much more numerous in the southern hemisphere than in the northern, both in terms of species and of individuals. Correspondingly, a greater proportion of southern than of northern hemisphere breeding seabird species make long migrations. Five (11%) of 47 species that breed north of the tropics extend to south of the tropics in the northern winter, whereas 14 (23%) of 61 species that breed south of the tropics extend to north of the tropics in the austral winter (calculated from maps in Harrison 1983). The implication is again that the sheer numbers of birds, in relation to the habitat available, influence the distances moved and area occupied outside the breeding season.
Very few migratory bird species have separate breeding populations north and south of the tropics. Examples include the Little Tern Sterna albifrons and Whiskered Tern Chlidonias hybridus in Asia-Australia, the Black Stork Ciconia nigra and Booted Eagle Hieraaetus pennatus in Europe-Africa, and the Turkey Vulture Cathartes aura and Black Vulture Coragyps atratus in North-South America.1 In all these species, the northern birds winter in the south when the southern birds are breeding but, among the terns, the southern birds also winter in the north when the northern birds are breeding. It is as though there is a single population of terns occupying the same range but with part of the population breeding at one end of the migratory terminal and another part at the other end. In each area, terns from each population can be distinguished according to whether they are in breeding or non-breeding plumage.
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