Maintenance of key pecking by response-independent food presentation: the role of the modality of the signal for food.

Three pigeons were exposed to a series of procedures in which periods of response-independent food presentation, on a variable-time schedule, alternated with periods in which food was never presented. The stimuli that signalled periods of food availability or non-availability varied from one procedure to the next, and were sometimes key colors, sometimes tones, and sometimes compounds of both. Key pecking was initiated and maintained when key color was a signal for food; key pecking was not initiated when a tone was the signal for food. However, control of key pecking that was already established could be transferred from key color to tone, and subsequently, initiated by the tone. It is suggested that for pigeons, pre-experimental relationships exist among food, visual stimuli, and pecking, and that a similar relationship, which includes auditory stimuli, must be induced in the laboratory.