Time-of-day discriminative learning in homing pigeons,Columba livia

Pigeons were trained on an operant procedure to discriminate between morning and afternoon when location did not vary (Experiment 1). The pigeons were placed on a fixed interval (FI) schedule in the morning and on a different FI schedule in the afternoon. Probe trials that occurred at the beginning of the training sessions were examined. The pigeons responded differently, depending on the time of day, reflecting the learning of a stable 24-h memory representation of the association between the FI schedules and the time of day. The pigeons from Experiment 1 were then clock shifted and tested twice, to determine whether they were relying on an endogenous circadian oscillator, an hourglass mechanism influenced by the photoperiod, or environmental noise to make the time-of-day discrimination (Experiment 2). The results of the second experiment indicated a circadian mechanism was most important for the observed time-of-day learning.