Experiments on Bird Orientation and Their Interpretation.

Summary. Among birds, the ability of sun‐azimuth orientation has been demonstrated experimentally in Starlings Sturnus vulgaris , Meadow Larks Sturnella magna , Barred Warblers Sylvia nisoria , Red‐backed Shrikes Lanius collurio and homing pigeons. Sun‐azimuth orientation consists in determining compass directions by the sun position regardless of time of day, hence mainly in the ability of correcting for the sun's daily azimuth displacement Evidence indicating the innateness of the underlying mechanism is not complete, but suggestive The estimation of the sun's speed must be considerably independent of personal experience. Individual Starlings differ in appraising the hourly angle of displacement in spite of their being constantly exposed to the natural sun The internal clock which is normally in phase with the astronomical day‐night cycle can be controlled and shifted by an artificial light‐dark cycle. The bird shows the deviation of its personal clock by producing determinate direction choices. The direction choices are equal to those produced by birds kept in outdoor conditions if the artificial light‐dark change is synchronized with the astronomical day‐night cycle. They deviate in a predictable manner if the artificial cycle is shifted. A shift by six hours results in a final directional deviation of roughly 90°. An exposure of 4–12 days to the phase‐shifted light‐dark cycle is sufficient to attain the full shift of the internal clock, which demonstrates that its “rigidity” is limited Recent advances in the problem of night orientation are reviewed. There is strong evidence of orientation by stars. The most convincing fact is oriented migratory activity under an artificial sky (planetarium) Recent facts are submitted supporting the ability of directed homing independent of familiar landmarks. At least in some areas the homing success depends upon the direction of displacement In the winter months a marked weakening of the homing ability of homing pigeons is observed in Northern Germany, at distances of 14–50 miles Overcast skies handicap both homing success and initial orientation at distances down to 7‐5 miles Day‐to‐day variations in homing success are observed which cannot be explained in terms of traditional meteorology No direct evidence for bi‐coordinate sun‐navigation is available. The hypothesis of sun‐arc navigation, based upon the observation of very short sections of the sun's path, is criticized theoretically. The main supporting experiments are re‐analyzed; one of them has been repeated and could hot be confirmed. On the whole the evidence in favour of this hypothesis is considered insufficient The explanation of homing orientation is still lacking. Prospects for a future experimental approach are discussed.