An Apparent Lunar Rhythm in the Day-to-Day Variations in Initial Bearings of Homing Pigeons
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In view of the suggestion from earlier experiments that detection of magnetic and gravitational stimuli may be linked, we investigated the possibility that gravitational changes across the synodic lunar month might influence the initial bearings of homing pigeons under conditions when naturally occurring fluctuations in the geomagnetic field have been shown to influence the birds’ orientation behavior. Six separate series of releases were conducted during four different years at three different sites. In all six, a nearly linear correlation between the pigeons’ mean vanishing bearings (MVB) and the day of lunar month (DLM) was found. The monthly oscillation of MVB could apparently exist stably in either of two modes, each 180° out of phase with the other; in some series the oscillation was from new moon to new moon and in others from full moon to full moon. Statistical analyses of several types support the notions that the best frequency of the oscillations of MVB is indeed almost exactly that of the synodic lunar month and that the turning points of the oscillations are the syzygies (new moon and full moon). However, attempts to tie the variations in MVB directly to the gravitational changes occurring during the lunar month have so far been unsuccessful; hence it is not yet known what environmental stimuli are acting as the immediate causes of the birds' changing orientation behavior.
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