The second major system of bird orientation makes use of the earth's magnetic field, which has both horizontal and vertical components. Imagine the earth as a hugely powerful magnet, whose north magnetic pole is situated fairly close to the geographic North Pole, and whose south magnetic pole is similarly close to the geographic South Pole.1 Running through the atmosphere between the two magnetic poles are invisible longitudinal lines of magnetic force, which circle the globe rather like the segments of an orange (Figure 9.6). At the equator, the magnetic force lines run horizontal to the earth's surface, but toward higher latitudes they dip more and more strongly into the earth, until they tip vertically downward at the magnetic poles. The vertical component of the field thus varies in strength within each hemisphere according to latitude. Hence, for any creature that can measure the inclination of the force lines, the earth's magnetic field can give a cue to latitude and direction (toward the equator or pole) within each hemisphere. However, it cannot give a reliable cue to longitude, so it cannot provide
1The current magnetic North Pole is situated at about 78°N, 103oW in northernmost Canada (Queen Elizabeth Islands), and the magnetic South Pole is at 65°S, 1390E (at the coast of the Antarctic continent at Wilkes Land). In wide regions around these poles, the horizontal component of the magnetic field is very small and the magnetic angle very steep, making magnetic orientation almost impossible there.
a firm basis for bi-coordinate navigation.2 Unlike celestial cues, the magnetic field can give consistent information in all weather conditions, both day and night, and unlike the sun compass, it needs no correction for time of day.
As revealed by radar studies, nocturnal migrants can occasionally orientate correctly even under completely overcast conditions, as can caged birds with no view of the sky. Caged birds lost this ability when isolated from both the sky and the earth's magnetic field behind metal-reinforced walls. When the magnetic field experienced by caged Robins Erithacus rubecula was rotated using a powerful electromagnetic coil so that, for example, magnetic north was shifted to the east while the field's total intensity and inclination, as well as other potential directional cues, were kept unchanged, the birds altered their orientation accordingly (Wiltschko & Wiltschko 1968). This crucial experiment showed conclusively that birds can respond appropriately to the earth's magnetic field, even though they may often use celestial cues.
Following from their early work, Wiltschko & Wiltschko (1972) found that Robins accustomed to the normal magnetic field in Frankfurt (0.46 gauss) initially gave random bearings in fields 26% lower (0.35 gauss) or 48% higher (0.68 gauss). However, if the birds were kept in the altered field for a few days, they could then orient in fields as low as 0.16 gauss or as high as 0.81 gauss. A similar sensitivity to altered field strengths was found in Whitethroats Sylvia communis and Garden Warblers S. borin (Wiltschko & Merkel 1971, Wiltschko & Gwinner 1974). It was suggested that the birds' ability to adapt to a range of field strengths would permit them to adjust to a range of magnetic intensities that they would encounter at different latitudes during migration.
The role of the geomagnetic field as a reference for migratory direction was further shown in experiments with several species of young passerines (Wiltschko & Wiltschko 1995, 1999), and more recently in a non-passerine (Gudmundsson & Sandberg 2000). Passerines were hand-raised without access to celestial cues and, when tested during autumn migration, they headed in their population-specific migratory direction with the magnetic field as the only cue (Wiltschko & Wiltschko 2003). These findings indicated that the geomagnetic field alone was sufficient for establishing the migratory course, at least in temperate latitudes. Birds tested at higher latitudes, where the angle of magnetic inclination was steeper, needed to have observed celestial rotation to adopt the correct heading
2Two magnetic parameters that vary in different directions across the earth's surface in a largely predictable way are the angle of inclination and the magnetic field strength, so in theory these parameters could be used for bi-coordinate navigation, at least in some regions. However, in other regions these two parameters coincide, so they do not form a grid system. Furthermore, in areas with geomagnetic anomalies, where the magnetic parameters can be largely distorted, and in areas close to the geomagnetic poles where the field lines are vertical, such a bi-coordinate map might be difficult to use. Further difficulty arises from the fact that geomagnetic parameters at a given location change through time, drifting significantly during the life of a long-lived bird. Even the positions of the two magnetic poles change somewhat over the centuries with respect to true geographical north and south. On a much longer timescale, the polarity of the geomagnetic field has changed several times in geological history (with north and south reversing), so polarity has not formed a consistent source of reference. The magnetic field is also prone to natural disturbances, such as solar flares.
in relation to magnetic cues alone (Weindler et al. 1996). Overall, the use of a magnetic compass has now been demonstrated experimentally in about 20 bird species, and its use may be widespread, but mainly in association with other information.
Trans-equatorial migrants using magnetic cues alone would face the problem that the magnetic field is horizontal for some distance north and south of the equator, and hence ambiguous with respect to direction. In the equatorial region, therefore, birds may have to use celestial cues to maintain an appropriate heading. Between the equatorial and polar regions, however, the earth's magnetic field represents a reliable and omnipresent reference for orientation, both in terms of directional and latitudinal information. No wonder, then, that various kinds of migratory animals appear to make use of it. Although the method of its sensory perception is still unclear, birds are sensitive to both inclination and intensity, but apparently not to polarity (Wiltschko & Wiltschko 1972).
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