Figure 20.3 Orientation directions of Blackcaps Sylvia atricapilla from two populations breeding east and west of a migratory divide in Europe, and from hybrids between these two populations. Within the inner circle, triangles indicate the parental population from southwest Germany (filled) and eastern Austria (open), while in the outer circle, dots show first-generation offspring of mixed pairs bred in aviaries. Arrowheads indicate group mean directions. The hybrids took mean directions intermediate between the two parental forms. Note the seasonal shift in direction shown in November by the southeastward migrating parents and the intermediate mean orientation of the hybrid offspring. From Helbig (1991b, 1996).
It is not sufficient that birds inherit a directional tendency that allows them to reach winter quarters. They must also be able to reverse this direction in spring in order to return to their breeding areas. Such reversal in direction has been demonstrated in tests of hand-raised Indigo Buntings Passerina cyanea,, Blackcaps and Garden Warblers (Chapter 12; Emlen 1969, Helbig et al. 1989, Gwinner & Wiltschko 1978, 1980). In autumn, Garden Warblers orientated southwest, changing some days later toward south or south-southeast, but in spring they headed towards the north, fitting with the loop migration recorded in wild Garden Warblers (Chapters 12 and 22).
Less direct indications for genetic control of migratory directions in various other species stem from the facts that: (1) inexperienced migrants tested in orientation cages showed regular species-specific and population-specific migratory directions; (2) displaced first-time migrants (White Storks Ciconia ciconia, Schuz et al. 1971, Starlings Sturnus vulgaris, Perdeck 1958) showed migratory directions parallel to those of their parental populations (Chapter 9); (3) partial and irrup-tive migrants also migrate regularly in specific directions (Chapter 18); (4) many young birds (notably cuckoos) migrate from Eurasia and tropical Africa independently of their parents (Chapters 9 and 15); and (5) inexperienced warblers and flycatchers show population-specific seasonal changes in their directional preferences in captivity matching the course of migration in wild birds. All these findings indicate that such migrants possess pre-programmed migratory directions or directional sequences, although they may, of course, deviate from their genetically prescribed course in response to local topography, weather and other conditions (Chapter 4). In some populations, ring recoveries of migrating birds lie in such a narrow and straight line between breeding and wintering areas that a simple clock-and-compass system might be sufficient to account for this pattern, providing that the birds are also able to correct for directional errors and wind displacements to stay on course (Mouritsen & Larsen 1998, Thorup & Rab0l 2001). Some radio-marked birds have also shown remarkably straight migration tracks (for Whooping Crane Grus americana see Kuyt 1992, for Common Eider Somateria mollisima see Desholm 2003, for Honey Buzzard Pernis apivorus see Hake et al. 2003).
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