Further experiments on the relationship between hippocampus and orientation following phase-shift in homing pigeons.

Following a clock- or phase-shift of the light dark cycle, hippocampal lesioned pigeons (Columba livia) consistently display a larger deviation in vanishing bearings away from the homeward direction compared to intact birds; an effect never seen in unshifted birds. In Experiment 1, control and hippocampal lesioned pigeons oriented similarly after being held 1 week under artificial lighting in the absence of a phase-shift. Housing under artificial light by itself does not result in between group orientation differences. In Experiment 2, control and hippocampal lesioned pigeons oriented equally well under overcast conditions, indicating that both groups had a functional magnetic compass. The between group difference in orientation following phase-shift does not appear to be a consequence of control birds being able to use both the sun and earth's magnetic field for orientation and the hippocampal lesioned pigeons only being able to use the sun. In Experiment 3, lengthening the time held under 6-h clock-shift from 1 to 2 weeks had no effect on the magnitude of the difference in orientation, but fast shifting produced clearer effects than slow shifting. Taken together, the data suggest that hippocampal lesions alter how a pigeon responds to a rapidly changing light-dark cycle, particularly following a fast-shift manipulation, suggesting an as yet unspecified relationship between the avian hippocampus and the circadian rhythm(s) that regulate sun compass orientation.