With increasing knowledge as to the winter distribution of species, and more especially subspecies of migratory birds, it becomes possible to determine more precisely the relationship between the summer-quarters and the winter-quarters of particular forms: this is a line of enquiry that is usefully supplemented by records of individual marked birds, and one that may throw interesting light upon the evolutionary origin of migrations.' (A. Landsborough Thomson 1936.)
One of the central tenets of ecology which has stood the test of time is known as Gause's (1934) Principle. It states that no two closely related species, which have identical requirements and are limited by resources, can persist together in the same area. Being different species, they are by definition unidentical, so inevitably one would be better adapted to exploit their mutual resources, and would eventually out-compete and eliminate the other. To coexist, therefore, closely related species must normally differ either in geographical range, habitat or foraging behaviour, or in more than one of these respects (Lack 1971, Newton 1998b). The relative importance of these segregation mechanisms in migratory birds may differ not only between species, but also between their breeding and non-breeding ranges. Two similar species may separate by geographical range at one season, perhaps, but occupy the same range and separate by habitat or feeding habits at another.
The notion that competition could have influenced the migration patterns of closely related bird species stems from the following observations: (1) some closely related species (and subspecies) segregate geographically in non-breeding as well as in breeding areas; (2) more species segregate geographically in winter, when food is scarcest, than in summer when food is more plentiful; and (3) species that coexist in the same geographical region (called sympatry) typically differ markedly in habitat or feeding behaviour, whereas species that occur in different regions (allopatry) often have similar habitat and feeding behaviour (for details and discussion see Lack 1944, 1971, Lack & Lack 1972, Cox 1968, Greenberg 1986, Newton 1998b). Allopatry arises, it is supposed, because individuals of each species survive or breed better in areas that lack closely competing species than in areas that have them (Salomonsen 1955, Cox 1968, Greenberg 1986, Gauthreaux 1982a). Hence, some common patterns of distribution that occur among migratory birds, or among migrants and residents, have often been attributed to the need to avoid competition, but the evidence is at best circumstantial, and other explanations often cannot be excluded. Nevertheless, all closely related species studied so far show some form of geographical or ecological segregation, except in unusual circumstances, such as temporary superabundance of food (Lack 1971).
Some closely related sympatric species occupy such different habitats that they are spatially segregated almost as effectively as if they occupied different geographical ranges. It is not just a matter of spatial scale, however: habitat differences can result in a patchwork distribution pattern, with areas of each species intermixed in the same region, whereas range differences result in the different species occupying totally (or almost totally) separate regions. It is these patterns of large-scale geographical segregation among migrants that are explored in this chapter.
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