Figure 13.5 Latitudinal shift between summer and winter distributions of bird species that breed in Europe. Includes wintering areas in Europe, Asia and Africa. From Newton (1995b).

the austral summer (northern winter). Take the west European migrants as an example. Some species move relatively short distances within Europe, but others move longer distances to Africa or southern Asia. But the net result, each autumn and spring, is a huge latitudinal shift in avifaunal distribution (Figure 13.5). In summer, the whole European assemblage of breeding birds is (by definition) concentrated north of 25°N, but in winter the same assemblage extends southwards as far as the southern tip of Africa (35°S). Forty-eight species of Palearctic birds reach the southern Cape of South Africa (Harrison et al. 1997), and some seabirds extend into the seas beyond. When they are in their wintering areas, the migrants add to the local species, increasing the overall species numbers, especially in the tropics (Chapters 23 and 24).

Many arctic-nesting species pass the northern winter in the southern hemisphere, including some populations of many shorebird species, three skuas Stercorarius, Arctic Tern Sterna paradisaea and other terns, Sabine's Gull Larus sabini, Peregrine

Falcon Falco peregrinus, Northern Wheatear Oenanthe oenanthe, White Wagtail Motacilla alba, Petchora Pipit Anthus gustavi and Red-throated Pipit A. cervinus. They gain the advantage of summer conditions year-round.

Trends within species

The above analyses (Figures 13.1-13.5) were based on the presence or absence of species at particular latitudes in winter. They were therefore based only on complete migrants, while for purposes of analysis partial migrants (in which only a proportion of individuals leave for the winter) were counted as year-round residents. However, in many species that breed over wide areas, a greater proportion of individuals migrate from higher than from lower latitudes. Thus, some such species in the northern hemisphere are completely migratory in the north of their breeding range and completely sedentary in the south, while in intervening areas some individuals leave and others stay (partial migration). European examples include Eurasian Blackbird Turdus merula and Peregrine Falcon Falco peregrinus, and North American examples include American Robin Turdus migratorius and Red-tailed Hawk Buteo jamaicensis. In general, therefore, the extent to which any population migrates for the winter broadly corresponds to the degree of seasonal reduction in food supplies. Taking account of partial as well as complete migrants, the latitudinal trends discussed above would be even more marked.

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