Responses to S- ("errors") are not a necessary condition for the formation of an operant discrimination of color. Errors do not occur if discrimination training begins early in conditioning and if S+ and S- initially differ with respect to brightness, duration and wavelength. After training starts, S-'s duration and brightness is progressively increased until S+ and S- differ only with respect to wavelength. Errors do occur if training starts after much conditioning in the presence of S+ has occurred or if S+ and S- differ only with respect to wavelength throughout training. Performance following discrimination learning without errors lacks three characteristics that are found following learning with errors. Only those birds that learned the discrimination with errors showed (1) "emotional" responses in the presence of S-, (2) an increase in the rate (or a decrease in the latency) of its response to S+, and (3) occasional bursts of responses to S-.
