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Oceanic Navigation: Are There Any Feasible Geomagnetic Bi-Coordinate Combinations for Albatrosses?
Susanne Åkesson and Thomas Alerstam
Journal of Avian Biology
Vol. 29, No. 4, Optimal Migration (Dec., 1998), pp. 618-625
Stable URL: http://www.jstor.org/stable/3677182
Page Count: 8
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The possibility that albatrosses use a geomagnetic bi-coordinate map for long-distance navigation at sea was investigated by evaluating how five different geomagnetic parameters (i.e. total field intensity, horizontal and vertical field intensity, inclination and declination) vary in areas where foraging albatrosses have been recorded by satellite telemetry in the Southern Ocean. Our objective was to investigate if any combination of these magnetic gradients was feasible as a geomagnetic bi-coordinate map. In particular we were interested in testing whether the bi-coordinate magnetic map hypothesis (i.e. gradient map based on total field intensity and angle of inclination; Lohmann and Lohmann 1996a) could explain the albatrosses' navigation system at these latitudes. Three satellite tracks were selected for Wandering Alba-trosses breeding in South Georgia in the south-western Atlantic and eight from the Crozet Islands in the south-western Indian Ocean. From the South American continent in the west (70°W) eastwards to the Kergulean Islands in the east (70°E), there is a considerable variation in the geomagnetic gradients. On the basis of our analyses we conclude: (1) There is no particular combination of two geomagnetic gradients that assigns every geographic location a unique set of co-ordinates in the South Atlantic Ocean. Hence, a gradient map could not explain the albatrosses' ability to navigate during their foraging excursions in the two breeding areas nor during their circumpolar non-breeding travels. (2) In subregions of the area under investigation, in particular around South Georgia, a useful grid was formed with a combination of two of the five magnetic gradients. In contrast, no such useful geomagnetic grid exists in the subregion around the Crozet Islands. (3) Because of irregular and weak gradients a bi-coordinate magnetic map seems highly unlikely to explain the albatrosses' navigation across major parts of the Southern Ocean. However, since we do not know the sensitivity of birds' magneto-receptors we cannot exclude the possibility that geomagnetic gradients in some way, perhaps in combination with other cues, are incorporated in the navigation system of albatrosses.
Journal of Avian Biology © 1998 Nordic Society Oikos