As indicated above, many birds change direction or accumulate extra fuel reserves part way through their journey, in preparation for a sea- or desert-crossing. Such changes in direction and fattening patterns observed at different stages of a journey may not depend solely on an inherent time programme. Experiments have indicated that birds can also respond to the star patterns or magnetic conditions found at particular regions en route (Chapter 9). In one experiment, Thrush Nightingales Luscinia luscinia were captured in southern Sweden at the start of their first autumn migration (Fransson et al. 2001, Kullberg et al. 2003). During the next 10 days, some individuals were exposed to the earth's local magnetic field (controls) and others to an artificial magnetic field typical in inclination and strength to that of northern Egypt, from where the birds are thought to depart for their Sahara crossing (experimentals). Fat deposition was significantly accelerated in the experimental birds, suggesting an effect of magnetic conditions on fattening. Whether this represented a specific response to the magnetic conditions of that particular area, or an unspecific response to a change in magnetic conditions, remains to be clarified. In another experiment, captive Pied Flycatchers Ficedula hypoleuca changed their directional preference at an appropriate date only when magnetic conditions (inclination and strength) were switched to those of the region where a directional change normally occurs, but not when magnetic conditions were held constant. This result suggested an involvement of both the endogenous timing mechanism and the 'expected' external cues; both had to be appropriate before a direction change occurred (Wiltschko & Wiltschko 2003). The implication is that birds have a built-in (genetically controlled) response to conditions in particular areas normally encountered on migration, which triggers an appropriate change in direction or fattening regime. If this is so, a similar dual mechanism could operate to signal arrival in winter quarters, but I know of no experiments that have tested this possibility.
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