The sun compass revisited.

Many animals, and birds in particular, are thought to use directional information from the sun in the form of a time-compensated sun compass, with predictably deviated orientation under clock shift being regarded as the litmus test of this. We suggest that this paradigm obscures a number of other ways in which solar-derived information could be important in animal orientation. We distinguish between the known use of the sun's azimuth to provide absolute geographical direction (compass mechanism) and its possible use to detect changes in heading (heading indicator mechanism). Just as in an aircraft, these two kinds of information may be provided by separate mechanisms and used for different functions, for example for navigation versus steering. We also argue that although a solar compass must be time-referenced to account for the sun's apparent diurnal movement, this need not entail full time compensation. This is because animals might also use time-dependent solar information in an associatively acquired, and hence time-limited, way. Furthermore, we show that a solar heading indicator, when used on a sufficiently short timescale, need not require time compensation at all. Finally, we suggest that solar-derived cues, such as shadows, could also be involved in navigation in ways that depend explicitly upon position, and are therefore not strictly compass-related. This could include giving directionality to landmarks, or acting as time-dependent landmarks involved in place recognition. We conclude that clock shift experiments alone are neither necessary nor sufficient to identify the occurrence of all conceivable uses of solar information in animal orientation, so that a predictable response to clock shift should not be regarded as an acid test of the use of solar information in navigation.