The combination of inherent time-distance programmes and directional preferences provides a viable mechanism to explain how juvenile birds migrating on their own can reach wintering areas unknown to them but specific to their population. Naïve autumn migrants do not therefore need to experience the particular conditions of their wintering areas before they stop migrating. After an appropriate time, caged birds from both European and North American breeding areas lost their autumn restlessness and fat reserves, even though they had moved no further than the confines of their cages. Moreover, when captive juveniles were experimentally transported to their species-specific wintering areas, or even beyond their normal wintering range, the migratory activity they showed in cages persisted as long as that of individuals kept in the breeding area (for experiments on young Garden Warblers Sylvia borin, Lesser Whitethroats S. curruca and Pied Flycatchers Ficedula hypoleuca see Gwinner 1971, Rab0l 1993). These birds also showed appropriate directional preferences. Similarly, juvenile Starlings Sturnus vulgaris and other species that were trapped on migration, flown by aeroplane and released immediately in the usual wintering areas for their population, or at some other locality off the normal route, resumed migration. Ring recoveries revealed that these transported birds moved in the same direction and covered about the same additional distance that they would have travelled had they not been displaced (Chapter 9; Perdeck 1964, 1967). These experiments again suggested control of timing and direction by an endogenous programme, rather than by location.
In a different type of experiment, juvenile Blue-winged Teal Anas discors were caught in autumn, held in captivity for some days, and released at the same place. Many then migrated after release, in the same direction but over shorter distances than normal (Bellrose 1958). This would also be expected if they were migrating to a time programme. While all these findings suggest that non-breeding destinations are broadly set by inherent time-distance and directional programming, this does not mean that birds are unable to move on if they encounter unsuitable conditions, or if they are homing to a specific site experienced in a previous year. Nor does it mean that all migratory species behave in this way, irruptive and other facultative migrants being more flexible in timing and distance (Chapter 18).
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