Entrainment of the circadian system of the house sparrow: A population of oscillators in pinealectomized birds

Removal of the pineal gland modifies the entrainment behavior of house sparrows. Abnormal entrainment occurs in pinealectomized sparrows exposed to 'skeleton' photoperiods (light cycles composed of 2 pulses of light per 24-h cycle). This abnormal entrainment depends upon the state of the locomotor activity (rhythmic or arrhythmic) before exposure to the light cycle, and upon the interval between the 2 pulses of light which constitute the skeleton photoperiod. The conditions that produce abnormal entrainment in pinealectomized birds are strongly correlated with those that produce 2 stable phases of entrainment to skeleton photoperiods in normal birds ('bistability phenomenon'). These results suggest that after pinealectomy, there remains a population of oscillators whose combined output is reflected in the locomotor activity of individual sparrows.