Solar cues in the migratory orientation of the Savannah sparrow, Passerculus sandwichensis

Results clearly implicate the setting sun as a critical source of directional information in the migratory orientation of the savannah sparrow, Passerculus sandwichensis. Savannah sparrows allowed a view of both sunset and stars displayed oriented behaviour in biologically meaningful directions during spring and fall seasons. When the same individuals were denied a view of sunset, and tested under the stars alone, disorientation characterized their behaviour. Furthermore, birds allowed a view of sunset, but tested under ‘overcast’ night skies (no stars visible), displayed well-oriented behaviour indicating the sufficiency of sunset. Experiments in which the migrant's internal chronometer was shifted suggested a fixed-angle (menotactic) response to the sunset cue rather than a time-compensating compass mechanism. I believe stars are valuable to this migrant as celestial reference points. Orientational information gained at the time of sunset is transferred to stars on a nightly basis. The relationship between solar and stellar cues is apparently hierarchical in the savannah sparrow. Information necessary to select the appropriate migratory direction is gained from the primary cue, the setting sun, while maintenance of that heading is dependent on a secondary cue, probably the stars.