Which of several potential compasses is most important to a bird at a given time has been investigated in so-called cue-conflict experiments, which involve presenting a bird with two (or more) orientation cues at once, manipulating one of them while leaving others unchanged, and monitoring the response of the bird (Able 1993). A typical experiment might involve placing an orientation cage surrounded by electric coils outdoors under a clear night sky. In this situation, the bird would have access to two known orientation cues: the stars and the magnetic field. The coils can be used to shift the direction of magnetic north so that magnetic compass directions differ from star-based ones. If, as compared to control birds tested in an unaltered magnetic field, the birds experiencing the cue-conflict changed direction in line with the magnetic field shift, one would conclude that in this situation magnetic information took precedence over stellar information.
Just such an experiment was conducted on three species of Sylvia warblers and European Robins Erithaca rubecula captured on migration (Wiltschko & Wiltschko 1975a, 1975b). When the directions of stellar and magnetic north were at variance, the birds seemed to orient preferentially with respect to magnetic cues. The warblers changed direction during the first test in the conflict situation, but the Robins did not shift until tested in cue-conflict for several consecutive nights (a finding later replicated on the same species elsewhere, Bingman 1987).
Cue-conflict experiments have proved especially useful in elucidating the cues used around sunset, a time when many birds set off on migratory flights. Orientation based on visual cues between the time of sunset and the appearance of the first stars could be based on the sun itself (e.g. the azimuth of sunset) or on patterns of polarised skylight which are particularly prominent at this time. Both could provide the same directional information, and both have the potential to indicate true compass directions. However, in cue-conflict experiments polarised light appears to be the predominant stimulus when placed in conflict with the sun's position or magnetic directions. When Yellow-rumped Warblers Dendroica coronata were exposed to rotated polarised light patterns for three nights, their subsequent orientation (apparently based on the sun, as indicated by mirror-shift tests) continued to exhibit the shifts induced by the polaroids (Phillips & Moore 1992). Further, when European Robins Erithaca rubecula and Blackcaps Sylvia atri-capilla were tested under depolarisers in a situation lacking directional magnetic information, they were disoriented even though sunset position was clearly visible to them (Helbig 1991a). Such findings point to polarised light as being a crucial visual orientation cue at dusk.
The findings from several cue-conflict experiments are listed in Table 9.2. Few have compared the same combination of cues, and in only two species (Savannah Sparrow Passerculus sandwichensis and Robin Erithaca rubecula) have all known cues been examined. In addition, different techniques and equipment were used in the experiments, further complicating comparisons. However, insofar as it is possible to generalise from the results, in short-term decision-making, magnetic cues took precedence over stellar cues, visual information at sunset overrode both of those stimuli, and polarised light (rather than sunset) was the relevant orientation cue used at that time.
Some studies produced results contrary to these general patterns, and possibly for good reasons. First, species may differ in the weight they give to different orientation cues, according to the types of journey they undertake. Second, individuals may also weigh cues differently, depending on their early experience. Third, while some birds are able to detect and respond rapidly to changes in some types of cue, they may take longer to assess and adjust to changes in other cues (as in the Robins mentioned above). In most of the experiments in Table 9.2, birds were tested for only one night or less, perhaps long enough for them to adjust to changes in some types of cues but not others. Experiments have shown the importance of recalibrating compass systems when birds are exposed to conflicting information. In nature, birds would not normally find themselves in situations where their various compass mechanisms suddenly gave disparate signals. Usually they could safely pool information from as many sources as possible in making orientation decisions.
In the wild, birds may regularly calibrate one compass cue against another. Thus, while the rotation of the earth relative to the sky provides a stable reference for defining geographic north and south, changing geomagnetic declination renders the earth's magnetic field less reliable as a geographic reference. Accordingly, young birds were found able to use celestial information to calibrate a migratory orientation response to the earth's magnetic field (Weindler et al. 1996, Bingham et al. 2003). The combined experience of the night sky and the natural geomagnetic field seemed crucial for songbirds at high latitude to find the appropriate migration direction to a population-specific wintering area (Weindler et al. 1996). Experienced adult Savannah Sparrows Passerculus sandwichensis also used celestial cues to recalibrate their migratory orientation to an experimentally
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