Bird migration as a world phenomenon has existed as long as birds have flown ... it is the specific circumstances that are so fascinating. (R. E. Moreau 1972.)
Variants occur around the basic outward-return movement between fixed breeding and wintering ranges, each in a limited number of species. They include: (1) moult migration; (2) long movements within the breeding season; (3) long movements within the non-breeding season, including irruptions and weather movements; and (4) nomadism. These variants can all be related to the particular ecological circumstances faced by the species concerned. They are all described in this chapter, but irruptions are discussed in greater depth in Chapters 18 and 19, because of the further light they throw on the role of food supplies in bird movements.
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