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The effect of supplemental feedings on pigeons' within-session changes in operant key-pecking was investigated. In different conditions, the feeding was either Purina Pigeon Checkers or mixed grain. The feedings were delivered 1, 4, and 12 hr prior to an experimental session. Responding changed significantly and systematically within sessions for all conditions. During the 1- and 4-hr pre-session feeding phases, absolute rates of responding were more suppressed and within-session decreases in responding were steeper when the reinforcer and supplemental feed were both mixed grain than when they differed. During the 12-hr pre-feeding phase, responding was unaffected by whether the supplemental food was similar to or differed from the food within the session. The results of the present experiment are generally consistent with the idea that habituation to the reinforce contributes to within-session decreases in operant responding (e.g., McSweeney & Roll, 1998).