Avian migration: the role of geomagnetic cues

Abstract: The regularity and scale of bird migrations has impressed mankind for generations. Birds rely on an inherited migratory program to guide them during the journey; however, recent evidence suggests that additional external cues are necessary. This thesis concerns how migratory birds use information from the Earth’s geomagnetic field, the role it plays in locating the breeding area and as a cue affecting fuelling behaviour.A displacement experiment with lesser whitethroats (Sylvia curruca) during spring migration indicates, for the first time, that birds include information from the geomagnetic field to locate their breeding area. Birds geomagnetically displaced south of the Swedish breeding area, exhibited consistent northerly orientation, close to the expected migratory direction. Birds displaced north, beyond their previous experience, failed to show a consistent direction of orientation. This suggests that they may have difficulty finding a direction in the absence of other compass cues.Migratory birds must obtain enough fuel and in the right places, e.g. before an ecological barrier, if they are to complete the journey successfully. Young birds on their first migration must do this without previous experience of the journey ahead. Inexperienced thrush nightingales (Luscinia luscinia) increased fuel deposition as expected when given a magnetic field of northern Egypt, their last stopover before the Saharan barrier. A single step change in magnetic field to Egypt resulted in the same change in fuel deposition as previously shown with multiple steps. In contrast, European robins (Erithacus rubecula) experiencing a simulated migration to the wintering area in Spain, where no large fuel loads are needed, showed low fuel deposition rates. This suggests that geomagnetic field information does not produce a general response in fuel deposition in a manner, similar to the way day length triggers migratory activity in autumn, rather birds respond in a biologically relevant way for each species.For both nightingales and robins the patterns of body mass change was unaffected by the time of season in birds experiencing the magnetic treatments. The magnetic field acts, therefore, as an important external cue, overriding the effect of season, helping birds make the right fuelling decisions along the migratory route. Furthermore, food intake was the major reason for the observed increase in fuelling rate compared to control birds. The finding that the magnetic field induces hormonal changes in thrush nightingales gives an indication of the underlying mechanism behind the food intake and resulting body mass changes seen in these experiments.