Relative frequency distribution of interresponse times (IRTs) using a bin size of 2 seconds averaged across birds in each season. Dashed line indicates the time at which IRTs were reinforced. Proportion of reinforced IRTs, those which occur after the 20 second delay, is shown to the right of the dashed line. IRT distributions are typically bimodal, with the most IRTs either short or long enough to be reinforced. N = 15. Error bars represent the 95% CI.