5.2 The Sun compass
The use of the Sun for orientation by birds was first demonstrated in the 1950s with European starlings (Sternus vulgaris). Mirrors were used to alter the apparent position of the Sun and the birds then orientated to the apparent position rather than orienting in the correct compass direction.
Can birds use the Sun alone as a compass guide?
The Sun on its own is not sufficient, because it moves across the sky during the day; so to maintain a constant direction using the Sun as a guide, a bird needs an internal clock.
The internal clock maintains a cycle of about 24 hours and is reset by the light-to-dark cycle of the environment. So, the Sun is playing two roles:
- it acts as a marker from which direction can be deduced
- its regular cycle keeps the internal clock accurate.
The importance of an internal biological clock for use of the Sun compass has been demonstrated by 'shifting' the clock.
For example, the bearings of the flight direction of rock pigeons (Columba livia, Figure 27) kept for several days in a room where the 'artificial' photoperiod was shifted by six hours either forward or backward, differed from those of non-shifted pigeons by about 90 degrees.