The sunazimuth compass

The height of the sun's arc in the sky varies with latitude and season, but it is always symmetrical with respect to true north or south. The use of the sun as a compass by birds has been known for more than 50 years. Under a sunny sky, Starlings Sturnus vulgaris kept in circular wire cages during the migration period oriented in the same direction as free-living birds. They varied the angle they took to the sun according to the time of day. If the sky became overcast, their directional preference disappeared. When their view of the sun's direction was changed using mirrors, the birds oriented at the same angle to the apparent sun as they would to the real sun (Figure 9.5). These experiments confirmed that Starlings made use of a sun compass which gave accurate information only if regulated by an internal clock, allowing adjustment of directional preference as the sun moved through the sky (the 'time-compensated sun compass'). Additional experiments in which Starlings were kept under artificial cycles of day and night of the same duration as natural time, but out of phase, revealed clearly that, in order to orientate, the birds used both the sun's position on the azimuth (direction from the observer) and the time of day. Given a simulated stationary sun, a caged migrant orientated at different angles to it according to the time of day.

The use of a sun-azimuth compass has now been confirmed experimentally in several species, including penguins walking over ice fields, and may be commonly used by diurnal migrants. Starlings and homing pigeons are able to employ the sun compass anywhere on the globe: under polar conditions when the sun does not set, and under equatorial conditions when it reaches its zenith. If birds of these species are transported to the southern hemisphere, however, they

Figure 9.5 Orientation of spontaneous diurnal migratory activity in a caged Starling Sturnus vulgaris under various conditions of sun exposure. The bird was tested in a pavilion with six windows during the spring migration season: (a) Behaviour under clear skies; (b) Behaviour under total overcast, when the sun was not visible; (c) Behaviour when the image of the sun was deflected 90° counter-clockwise by means of mirrors; (d) Behaviour when the sun was deflected 90° clockwise by means of mirrors. Each dot represents 10 seconds of fluttering activity. Dotted lines show incidence of light from the sky; arrows denote mean direction of activity. Redrawn from Kramer (1951).

Figure 9.5 Orientation of spontaneous diurnal migratory activity in a caged Starling Sturnus vulgaris under various conditions of sun exposure. The bird was tested in a pavilion with six windows during the spring migration season: (a) Behaviour under clear skies; (b) Behaviour under total overcast, when the sun was not visible; (c) Behaviour when the image of the sun was deflected 90° counter-clockwise by means of mirrors; (d) Behaviour when the sun was deflected 90° clockwise by means of mirrors. Each dot represents 10 seconds of fluttering activity. Dotted lines show incidence of light from the sky; arrows denote mean direction of activity. Redrawn from Kramer (1951).

orient themselves incorrectly, interpreting the sun as if they were in the northern hemisphere (i.e. indicating south rather than north). Regular trans-equatorial migrants must presumably be able to make the necessary adjustment, but how they do so is still unclear (Schmidt-Koenig et al. 1991). If birds adjusted for time of day on the basis of their experience in the previous few days, their response would be appropriate for the latitude and time of year (which also influence sun position).

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