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We explored pigeons' ability to leam a particular sequence of stimuli in which the durations of each stimulus varied among trials, where the first response at the end of the sequence was reinforced. In Experiment 1A, we found that pigeons failed to use the whole sequence of three stimuli to predict food reinforcement, and instead responded only to the third, "rewarded" stimulus. When rewarded (1-2-3) and nonrewarded (2-1-3) sequences were used in a go/no-go procedure in Experiment 1B, however, pigeons showed a tendency to rank-order responding, with higher response rates to the second than to the first stimulus, as well as lower response rates to the third stimulus on nonrewarded-sequence trials. In Experiment 2, pigeons showed rudimentary rank-ordering of five stimuli in sequence, with lower responding to the final stimulus on nonrewarded trials, even when the sequence presented differed from the rewarded sequence only in a reversal of the second and third stimuli. Pigeons were capable of using ordinal information in a temporal task, but only when that information was easily discriminable and led to explicit consequences (i.e., rewarded vs. nonrewarded sequences).
