Migratory Orientation in the Indigo Bunting, Passerina cyanea. Part II: Mechanism of Celestial Orientation

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Abstract
The Zugunruhe orientation of caged indigo buntings was tested under artificially manipulated planetarium skies during the fall of 1964 and the spring and fall of 1965. When exposed to skies advanced and retarded 3, 6, and 12 hr. from local time, the buntings generally continued to maintain their normal migration direction. This implies that the birds were not relying upon a bicoordinate celestial navigation system; neither were they employing a form of time-compensation analogous to that proposed in sun-compass orientation. The birds make use of additional information provided by the existence of numerous stars and by the constant, 2-dimentional, spatial relationships which exist between them; they determine the migration direction by responding to Gestalt stimuli provided by star patterns. Attempts to determine which configurations are of especial importance met with only partial success. Although considerable variation seems to exist in the specific cues used by different individual buntings, results of blocking experiments generally indicated that the northern celestial sky was especially important during both migration seasons. Both theoretical considerations and empirical evidence further suggested that the circumpolar area within 35 degrees of the North Star may be important if not essential to this orientation process. More detailed conclusions must await further experimentation on a larger sample of birds.