Pigeons acquired a successive depayed matching-to-sample task at delay intervals ranging from 2.5 to 7 seconds. Test sessions were conducted during which delay-interval illumination conditions were changed from those illumination conditions that prevailed during the baselines. Compared to baseline delayed matching performance, changing delay-interval illumination disrupted matching. This disruption occurred whether the change in delay-interval illumination represented an increase or a decrease, relative to the baseline, and whether there was or was not a change in illumination during the test session. It was concluded that illumination per se introduced during delay intervals of delayed matching tasks does not interfere with pigeon short-term memory. Rather, a change in delay-interval illumination, relative to the baseline, appears to retroactively interfere in pigeon short-term memory.