This paper is concerned mainly with the differences between obligate and facultative migration in birds. Obligate migration is considered “hard-wired”, in that the bird seems pre-programmed to leave its breeding area at a certain time each year, and to return at another time. Timing, directions and distances are relatively constant from year to year. This type of migration is thus characterised by its regularity, consistency and predictability. It is found in both short-distance and long-distance migrants, but mainly in the latter. In contrast, facultative migration is considered optional, occurring in response to conditions at the time. Individuals may migrate in some years but not in others, depending on the prevailing food supplies or weather conditions. The timing of autumn migration, and the distance travelled, can be highly variable between individuals and, at the population level, between years. Facultative migration is typical of many partial migrants, but is found in its most extreme form in so-called irruptive migrants. While individual obligate migrants typically return to the same breeding localities year after year, and sometimes also to the same wintering localities, individual irruptive migrants typically breed or winter in widely separated areas in different years, wherever conditions are favourable. It is suggested that these two types of migration are best considered not as distinct, but as lying at opposite ends of a continuum of variation in bird migratory behaviour. Both systems are adaptive; one to conditions in which resource levels vary regularly and predictably in space and time, and the other to conditions in which resource levels vary unpredictably. Suggestions are made for experimental work on captive irruptive species
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