All the species discussed above - whether owls, raptors or waterfowl - show variable migration distances, moving towards milder climes in autumn, often in several stages, separated by weeks or months of residence. Some such species also migrate variable distances in spring, partly because many individuals settle in different parts of the breeding range each year, depending on conditions encountered en route. This flexible settling behaviour differs strikingly from that of other migrants, in which individuals occupy fixed breeding and wintering areas from year to year, but it is adaptive for the species concerned, which face unpredictable variations in habitat or feeding conditions from year to year, in either breeding or non-breeding seasons, or both. In contrast to obligate migrants, whose behaviour is apparently under stricter genetic limits (Chapter 12), and hence more consistent from year to year, facultative migrants are more responsive to prevailing conditions. Again, the two types are not wholly distinct, but lie at different points on a continuum of behaviour from fixed to flexible, in adaptation to a spectrum of resource conditions, and the same species may behave differently in different parts of its range. A further extreme in behaviour is evident in the deserts of Australia, where some rodent-eaters, along with waterfowl and other birds, are more appropriately classed as nomadic in response to sporadic rainfall, as described in Chapter 16.
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