On current thinking, then, the urge to migrate in autumn and spring, at least in obligate migrants, is genetically controlled. It is reflected in an autonomous rhythm of physiology and behaviour, which is kept on schedule by daylength changes (Gwinner 1972, 1986, Berthold 1996). This inherent system controls both fattening and migratory restlessness, as well as the general direction and time-course of migration. Most of the experimental work which gave rise to this paradigm concerned naive juvenile birds that had no previous knowledge of the wintering range of their population. The situation differs somewhat in experienced birds migrating to a known site, as first shown in the experiments with Starlings Sturnus vulgaris and others described in Chapter 9. Adult birds displaced off their normal route in autumn could change direction and find their previous wintering areas, whereas juveniles continued in the usual direction after displacement, making no correction and moving parallel to the normal route. Having no previous experience of the wintering area, juveniles evidently used one-directional orientation, maintaining their displacement. However, on return migration to their breeding areas, both juveniles and adults proved able to return to the region of their birth (Chapter 9; Perdeck 1958). Previous experience of the breeding area enabled them to compensate for their displacement, and find their way home.
These early findings on the role of experience were extended by additional work on Dark-eyed Juncos Junco hyemalis, in which adults usually return to the same breeding and wintering sites in successive years. Individuals caught in July on their breeding areas and transported to a locality within the wintering range showed the usual autumn restlessness and fattening (Ketterson & Nolan 1986). On the other hand, birds caught in winter and held at the capture site over summer showed no migratory restlessness and fattening during the next autumn.
This held for birds kept on local 'wintering area' photoperiods, and for birds kept on wintering areas, but exposed to longer photoperiods typical of their breeding areas. This suggested that the locations of these birds on their previous wintering sites suppressed their autumn migratory behaviour. Both groups showed migratory behaviour in spring.
A further experiment involved a similar procedure, except that Juncos were kept over summer in their wintering areas, and then released in September, the normal time of autumn migration. Of 129 birds released, 47 were subsequently re-sighted in the release area, often within the same home range where they had been caught in the previous winter. Hence, these birds, which were already in wintering sites known to them, did not migrate that autumn. However, they apparently did migrate in the following spring (as none were found in wintering areas), and in the next autumn (when some re-appeared on their winter ranges).
The holding of Juncos on a previous (known) wintering site was apparently sufficient to suppress the usual autumn migration (Ketterson & Nolan 1986). It was uncertain what happened to the birds that were not re-sighted, but if they had dispersed only a short distance from the release sites, they would not have been found. These experiments thus provided further indication that the endogenous template of migration could be altered by experience. Similar results were obtained with migrating Dunnocks Prunella vulgaris in Europe (Schwabl et al. 1991b).
The same may hold for the spring return to familiar breeding areas, whether by juveniles or by adults, with migratory behaviour ceasing only when the birds have reached their specific goal. Dark-eyed Juncos that were held captive in wintering areas for up to two months beyond the normal spring departure date disappeared (presumably migrated) upon release, even though their gonads were by then in full breeding condition (Wolfson 1942, 1945). Likewise, among various warblers and finches that were caught in spring and held captive at a locality en route, migratory restlessness and appropriate directional preferences continued for up to several weeks longer than normal (Merkel 1956, Shumakov et al. 2001). Bramblings Fringilla montifringilla caught on spring migration, but south of their breeding range, also showed northeast directional preferences until late August, long after their spring migration would normally have finished.
These various findings could be interpreted as continuing attempts by these birds to reach their breeding areas, again indicating the importance of locality in suppressing the migratory behaviour of experienced birds returning in spring to a familiar area. This is different from the simple vector migration shown by inexperienced juveniles on their first autumn migration, as described above. It may account for the fact that some other birds caught from the wild and tested in captivity continued spring restlessness well into summer, or autumn restlessness well into winter, much beyond the normal migration seasons. They could have been birds held far away from their previously experienced breeding or wintering sites.
That spring migration can be suppressed by recognition of a familiar area was shown in an experiment with Indigo Buntings Passerina cyanea (Sniegowski et al. 1988, Ketterson & Nolan 1990). Males were caught on their nesting territories, held over winter, and released there in spring at a date when migration was just beginning in conspecifics wintering far to the south. Controls were transported and released 1000 km to the south. Seven out of 20 buntings released in spring on their nesting territories remained, while eight out of 20 released to the south returned to their nesting localities. The remaining birds in each group were unaccounted for. Nevertheless, these results indicated that, when migrants were exposed before spring migration to their previous nesting place, they did not migrate.
During reintroduction projects at various localities in western Europe, White Storks Ciconia ciconia were reared and kept in aviaries for at least one winter, which prevented them from migrating as they normally would. After they had been released, they remained in the same areas year-round, breeding and wintering there, and supported by supplementary food in winter. However, the free-living offspring of these birds migrated as normal for their population, yielding ring recoveries along the usual southwestern migration route (Fiedler 2003). These findings provided further indication that experience based on learning and memory can modify the inherent migratory behaviour of individuals. The findings from these different types of experiment on the role of experience on migration behaviour are summarised in Table 12.1.
Some remarkable feats of memory became evident in some radio-tracked birds which took the same routes in successive years, even to the extent of repeating apparent mistakes. For example, a Lesser Spotted Eagle Aquila pomarina in successive autumns took the same diversion off its route and back again, which added an apparently unnecessary 500 km and 2-3 days to its migration (Meyburg et al.
Table 12.1 Results of some trap and retention experiments (see text)
Held over first Did not winter in breeding subsequently area migrate
Held in wintering area over summer and then released
Held in breeding area over winter and then released
Held on spring migration route
Held on spring migration route and released about two months late
Did not migrate but stayed in wintering area until spring
Did not migrate but stayed in breeding area until autumn
Showed prolonged migratory restlessness
Left site, presumably on migration
Fixation to former wintering area greater than motivation to migrate in autumn
Fixation to former wintering area greater than motivation to migrate in autumn
Fixation to former breeding area greater than motivation to migrate in spring
Failure to reach breeding area resulted in migratory restlessness beyond the normal period Failure to reach breeding area resulted in prolongation of migratory behaviour
White Stork Ciconia ciconia
Dark-eyed Junco Junco hyemalis
Indigo Bunting Passerina cyanea
Dark-eyed Junco Junco hyemalis, Chaffinch Fringilla coelebs, Brambling Fringilla montifringilla
Dark-eyed Junco Junco hyemalis, Brambling Fringilla montifringilla
Ketterson & Nolan (1986)
Sniegowski et al. (1988), Ketterson & Nolan (1990)
Merkel (1956), Shumakov et al. (2001)
Wolfson (1942, 1945), Shumakov et al. (2001)
2002). This bird seemed to have remembered and repeated the same detour from one year to the next.
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