The large-scale movements of birds can conveniently be divided into dispersal, dispersive-migration, migration, irruption and nomadism, although these different types of movements intergrade with one another, and the same populations may show more than one type of movement at different stages of their annual cycle. This book is concerned with all these types of movements, but chiefly with migration, defined as a seasonal return movement in fixed directions between separate breeding and wintering ranges. Migration occurs to some degree in most species of birds that live in seasonal environments. It leads to massive twice-yearly changes in the distributions of birds over the earth's surface.
Some migratory birds travel relatively short distances of a few tens of kilometres between their breeding and wintering areas, but others travel hundreds or thousands of kilometres, sometimes crossing long stretches of sea or desert or high mountain ranges, where they cannot rest or feed. They accumulate large reserves of body fat for the journey. Such birds show impressive navigational skills which enable individuals to return to the same breeding and wintering sites year after year. Although migration occurs mainly on a north-south axis, many species have a strong east-west component in their journeys, especially those that move from the seasonally hostile centre to the milder edges of the northern land masses. Individuals in so-called sedentary populations mostly move over short distances of at most a few tens of kilometres, and show no directional preferences, so that the population occupies essentially the same range year-round. Only one bird species is known to hibernate through the unfavourable season.
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