Magnetic cues: are they important in Black‐browed Albatross Diomedea melanophris orientation?

Procellariiformes are well known for their excellent homing abilities (see references in Matthews 1968 and Dall’Antonia et al . 1995, Warham 1996). Albatrosses, in particular, have a reputation as skilful oceanic navigators since they can cover enormous distances during foraging flights and pinpoint a specific remote island where their colonies are located. A crucial experiment carried out on Laysan Albatrosses Diomedea immutabilis (Kenyon & Rice 1958) showed that these birds managed to home to their colony after a passive displacement of 2116–6629 km in the Pacific Ocean. The masterly navigation abilities of albatrosses clearly involve a high-precision navigation system. Many studies have previously described the biology of different species, but, to our knowledge, only a few have focused on the mechanism used to achieve such extraordinary performances. The two main hypotheses proposed are egocentric navigation principles (path integration) as in hymenopterans, and true navigation (see Papi 1992 for references). True navigation postulates the existence of a map (large-scale bi-coordinate maps) and compass (solar, magnetic) mechanism (Wallraff 1990). Evidence obtained from other birds suggests that sensitivity to the earth’s magnetic field might be involved in the orientation mechanisms of albatrosses. Several species of migratory passerine birds use a magnetic compass during the migratory trip while homing pigeons are reported to be disorientated if carrying magnets or in sites where magnetic anomalies exist (see Wiltschko & Wiltschko 1996). Magnetic fields could be important for sea turtles also. Like albatrosses, sea turtles are excellent oceanic navigators, and move in a uniform environment where the lack of landmarks eliminates their implicit capacity to guide animals (Carr 1984, Papi & Luschi 1996). Hatchling Loggerhead Sea Turtles Caretta caretta can detect two geomagnetic parameters: the angle of inclination and the total field intensity (Lohmann & Lohmann 1994, 1996a, 1996b, 1998). These two parameters, varying along gradients across the earth’s surface, would provide turtles with a bi-coordinate map useful to determine their position relative to a goal area. However, the only attempt to test the geomagnetic navigation hypothesis in migrating adult Green Turtles Chelonia mydas failed (Papi et al . 2000). A geomagnetic bi-coordinate mechanism has been proposed for birds but never demonstrated (Wiltschko & Wiltschko 1996). Considering this possibility Åkesson and Alerstam (1998) have recently investigated whether any combination of different geomagnetic parameters forms a reliable bi-coordinate map suitable for Wandering Albatrosses Diomedea exulans to navigate at sea. Their findings indicate that, in some areas, the use of a magnetic gradient map could not be possible although they do not exclude the existence of such a system used elsewhere. The first step to investigating whether this parameter has a role in albatross orientation is to interfere with the bird’s perception of the Earth’s geomagnetic field. The simplest way to achieve this is to perform homing experiments with birds equipped with magnets that alter the animal’s perception of the magnetic fields around it (e.g. Keeton 1971, Wallraff & Foà 1982, Ioalé 1984, Moore 1988, in homing pigeons and Massa et al . 1991 in Calonectris diomedea ). If magnetic field is important in albatross orientation, a bird deprived of the correct perception could perform poorly during foraging trips. We report the results of an experiment in which we recorded the foraging performance of Black-browed Albatrosses Diomedea melanophris carrying magnets.