Orientation of Nocturnally Migrating Swainson's Thrush at Dawn and Dusk: Importance of Energetic Condition and Geomagnetic Cues
We studied the early morning cage orientation of nocturnally migrating Swainson's Thrushes (Catharus ustulatus) during three fall migration seasons. The results were compared with earlier free-flight release tests under starry skies and were found to be consistent with continuation of migratory flights in the expected seasonally appropriate direction. Energetic condition proved decisive: fat birds chose directions in accordance with migration across the Gulf of Mexico, whereas lean birds oriented away from the coast, possibly in search of habitats suitable for refuelling. Whereas the orientation of fat Swainson's Thrushes was affected by experimental shifts of the magnetic field, the response during morning tests was larger than expected. A parallel series of orientation cage experiments performed during evening twilight showed a response to deflected magnetic fields that was close to the expected shift, which suggests a difference in integration of directional information between early morning and evening twilight activity. However, within-individual response to deflected magnetic fields was of the same magnitude during both morning and evening tests.