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population of high density must transfer itself en bloc to an entirely different part of the globe. This movement is totally different from a " dispersal" of birds after the breeding season, where a high density of population is merely dissipated throughout the surrounding area, and does not transfer itself as a population. Examples are afforded by the herring-gull and the lesser black-backed gull. These two birds breed in Britain under similar conditions and in more or less coincident areas, yet in the autumn, the herring-gull scatters around our coasts and may be found there during the winter and rarely goes more than 200 miles beyond its breeding area. The British lesser black-backed gull, on the other hand, truly migrates, reaching the northern tropics and is a comparative rarity in this country in winter. Thus in true migration we get a real abandonment of a summer area in favour of a winter habitation. In addition, this movement must be regular ; the sudden " irruptions " of birds like crossbills, waxwings or nutcrackers from the northern pine forests when the cone and berry harvests fail there, cannot be classed as true migration. |
