Ardea
Official journal of the Netherlands Ornithologists' Union

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Cavé A.J. (1982) Experiments on the use of the sun by Starlings in the discrimination of geographical locations for navigation. ARDEA 70 (2): 197-216
To gain more insight into goal orientation (navigation), experiments were designed to separate the process of location finding (the 'map') from the process of direction finding (the 'compass'). These experiments dealt only with discrimination between two locations, in this study lying 237 km apart in an east-west direction. These locations were The Hague (52¦03'N 4¦11'E) called H, and Mnnster (52¦04'N 7¦39'E), called M. Caged starlings were trained to use two perches, each perch at a different location, i. e., one perch in H and the other in M. Additional tests were carried out near Arnhem (52¦N 5¦53'E) which is situated roughly halfway between Hand M. All tests and training sessions were carried out at a fixed solar time, i.e., that of Arnhem. Tests at the training sites in Hand M with full view of the surroundings (known landscape) gave the following results: a) a significantly different choice was made at the two locations, with a preference for the correct perch; b) an effect of the visibility of the sun on the birds' performances, which were better with the sun directly visible or unlocalizable than in the intermediate situations, i. e., with the sun indirectly visible or the sun-side of the sky lighter. This suggests that, if available, the sun was used in discrimination between Hand M at the training sites and that the birds resort to the use of well-known landmarks when the sun cannot be located. Both in Hand M, the birds were also tested at other sites (unknown landscape). Under sunny conditions at least some of the birds were able to discriminate between Hand M. Under overcast conditions their performances were poorer, which suggests that the sun is used in the discrimination process. The effect of the visibility of the sun was further investigated in experiments in which the sun was screened off. Performance was poorer in the sun-screened than in the unscreened situation. This too indicates that the sun was used in discrimination between Hand M. During tests in Arnhem (halfway between Hand M) the birds showed a preference for the perch that had been the correct one during the preceding training. This effect of previous training was greater under overcast than with the sun visible, which suggests that Arnhem was not some unknown location for the birds, but was related to Hand M. This resuits provides another indication that the sun is used in discrimination between locations. Hand M were not recognized when the birds had been trained and tested at the training sites with the landscape screened off. Under these conditions no effect of the visibility of the sun was found. From this experiment it was concluded that the sun is used in combination with a parameter derived from the landscape, e.g., the horizon. The experiments described in this paper dealt with longitudinal discrimination, i.e., comparison of time-shifted sun arcs in he case of sun orientation. This is only possible if the birds have at their disposal a 'clock' which is independent of local time. All training sessions and tests were carried out at a fixed local (sun) time for Arnhem (which is situated halfway between Hand M). This provided the birds with an external chronometer, and the results could not indicate whether they have a 'chronometer' of their own. However, the birds used the sun as a cue in preference to other cues that are stationary in time. This indicates that the sun forms part of the normal system used in discrimination between locations, and thus supports the view that sun navigation is one of the means that birds have at their disposal for goal orientation.


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