The orientation of night-migrating passerines without the directional influence of the starry sky and/or the earth magnetic field.

During four seasons in 1972-73 a total of 598 orientation experiments -- including 157 displacements -- was carried out. The purpose of the experiments was to investigate the orientation under an overcast sky and/or when the earth magnetic field was screened by a strong artificial magnetic field. These experiments were compared with experiments carried out under the presumed maximum condition, i.e. under a starry sky and without magnetic disturbances. In general, little or no "reduction" of the sample mean vector, was observed under the "deprived" conditions. However, the concentration of the individual mean vectors and the activity were lowered. Clear compensation for a displacement was found in the absence of the stars and in a disturbed magnetic field. This means that directing cues from other sources than the stars and the earth magnetic field were involved. It is guessed that inertial orientation plays an important role. Reverse orientation -- including reverse compensation -- was found fairly frequently. It is supposed to be correlated with overcast, and perhaps in general stress conditions. In conclusion, simple one-direction orientation models are not thought to be sufficient to "explain" the observed orientations.