not only a directional sense, but also a map sense: an ability to determine their spatial position with respect to home (or at least their direction towards home) based solely on information from their current location. Among other things, such map-based navigation would enable birds to correct for wind-induced drift off their normal migration route.
The fact that all these birds, whether caught on breeding, wintering or migration areas, travelled approximately the same distance as they normally would, has been attributed to the involvement of an internal clock, which ended migratory behaviour after an appropriate period (Chapter 11). Transportation of other birds to a suitable wintering locality did not switch off the behaviour immediately, only after the lapse of an appropriate time period. This was true even of young Pied Flycatchers Ficedula hypoleuca and Garden Warblers Sylvia borin which were taken from Denmark to Kenya (south of the normal wintering area) (Rab0l 1993). The same behaviour would not necessarily be expected in adult birds, which had experienced the area previously (Chapter 12).
Further evidence for inherent directional preferences came from other early field experiments, in which young birds of various species were held in their natal areas until all other individuals of their species had left. After their release, the young were found from subsequent sightings and ring recoveries to have migrated in the direction normal for their population. For example, 144 White Storks Ciconia ciconia were taken from nests in the Baltic region and reared in western Germany. Released after all local migrant storks had passed, these birds showed a strong tendency to head south-southeast, the appropriate direction for the population from which they were drawn, but quite distinct from the southwest direction taken by storks from the release area (Schuz 1949).
Similarly, 1071 young Blue-winged Teal Anas discors were caught on migration through Illinois, and held until November, when the species had virtually left the USA for South America. Half of the delayed young birds recovered in the same winter were within 40 km of the release point, but the other 54 were almost exclusively in the southeast sector normal for their species (Bellrose 1958). These experiments eliminated the possibility that the released birds may have been influenced by other individuals of their species, because other individuals had already left the release area. More stringent tests of innate directional preferences involved the release of young birds in areas where their species does not breed. Thus, 21 White Storks were transported from the Baltic region for release in England (Schuz 1938), 953 Herring Gulls Larus argentatus were transported from the Friesian Islands for release in inland Germany (Drost 1955, 1958), 192 White Storks were transported from Algeria for release in Switzerland (Bloesch 1956, 1960), and 377 Blue-winged Teal ducklings were transported from Minnesota for release in Missouri (Vaught 1964). Again ring recoveries from all these birds indicated migration in the direction normal for the population from which the birds were drawn.
More recent displacement experiments have used smaller numbers of birds, and tested their directional preferences in orientation cages. The results were inconsistent, as some young birds showed no change in directional preference after displacement (and thus, like the experiments above, supported the vector hypothesis, Hamilton 1962a, Mouritsen & Larsen 1998), while others appeared to change their preferred direction so as to correct for displacement, taking a direction that would lead them to some place on the normal migration route (Rab0l 1969a, 1994, Akesson et al. 2001, 2005, Thorup & Rab0l 2001). It is possible that not all the experimental birds were in a migratory state when tested, or that the distance, direction, speed or manner of displacement influenced the results. The most impressive directional corrections were shown by long-distance migrants that were displaced slowly, predominantly east-west, whereas the Starlings and others mentioned above were transported rapidly over shorter distances, and mainly north-south. Both adults and juveniles were involved in the long-distance displacements, and both corrected appropriately. Similar findings were obtained when naturally drifted migrants (including juveniles) were tested in orientation cages, or tracked by radar (Evans 1968, Chapter 10), and when birds were subjected to simulated displacement in a planetarium (Rab0l 1992). The difference between adults and juveniles found in the earlier experiments was not therefore borne out in all subsequent ones, especially with substantial east-west displacements.
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