The existence of innate migratory directions is further supported by those species in which juveniles migrate to wintering areas independently of adults. A striking example is provided by the European Cuckoo Cuculus canorus, in which the last young leave the breeding areas up to a month after the last adults have left, yet they still find their own way to their African wintering areas, despite their various foster species wintering in a range of different areas. The same is true for other cuckoos, including the Shining Bronze Cuckoo Chrysococcyx lucidus, which migrates from New Zealand 4000 km over open water to winter on small Pacific Islands.
Yet other evidence for an innate directional preference comes from the ring recoveries of migrants, which in many species lie on a straight line route from the breeding area towards a staging or wintering area (for Pied Flycatcher Ficedula hypoleuca see Mouritsen 2001, for Linnet Carduelis cannabina see Newton 1972, for Honey Buzzard Pernis apivorus see Thorup et al. 2003). Ring recoveries tend to fan out to some extent with increasing distance, but this would be expected assuming a normal distribution of individual directional preferences. Similarly, radio-tracking has shown that, in some species, each step of the migration follows a rather straight track, with only slight and temporary deviations from the overall direction (for various thrushes see Cochran et al. 1967, for Whooping Crane Grus americana see Kuyt 1992, for Osprey Pandion haliaetus and Honey Buzzard Pernis apivorus see Thorup et al. 2003). Other species, however, show much greater variation in directions between individuals, and less step-to-step consistency within a journey (Chapter 18). Final confirmation of the genetic determination of migratory directions came from the cross-breeding experiments reported in Chapter 20, in which juveniles showed directional preferences intermediate between those of their parents.
Because some adult birds migrate between specific breeding and wintering territories, separated by hundreds or thousands of kilometres, they cannot rely solely on simple clock-and-compass navigation, which is far too imprecise. The navigation strategy of experienced adult birds must include a precise map component, based on either local or global cues. Both juveniles and adults may have the capacity for bi-coordinate navigation, but juveniles may use it only when they have learnt the coordinates of particular breeding and wintering areas, or when they have drifted far off course. The precise mechanism of goal-oriented navigation remains unknown, although various hypotheses have been proposed (for discussion see Wallraff 1991, Berthold 1993).
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