Stimulus stringing by pigeons.

Pigeons were trained to peck one, two, three, and then four colors in a predetermined sequence from a five-key array where, over trials, each color appeared equally often in each position of the array. Incorrect pecks resulted in a buzzer and trial termination, with the same array presented for the next trial. Correct pecks produced feedback and correct strings could produce food. All subjects performed at a high level of accuracy with no difference at asymptote between a continuous and a mixed spectral sequence as the required order. Transfer to a new set of arrays had little effect on accuracy. Errors forward in the sequence had the highest probability, followed by repeat errors, backward errors, and dark-key errors. Some arrays had a higher level of accuracy than others but a corresponding systematic variable could not be identified.