In nature animals constantly encounter novel stimuli and need to generalize from known stimuli. The animal may then learn about the novel stimulus. Hull (1947) suggested that as they learn animals distinguish knowledge based on direct experience from inference by generalization and in support of this view suggested that if a subject is directly trained to a stimulus subsequent extinction of responses is slower than when the response is based on generalization. Such an effect is also predicted by Bayesian models that relate the rate of learning to uncertainty in the estimate of stimulus value. We find support for this prediction when chicks learn about a novel colour (orange) if the initial evaluation is based on similarity to known colours (red, yellow). Specifically, if an expected food reward is absent the rate of extinction of the response to the novel stimulus exceeds that for the familiar colours. Interestingly, the change in relative preference for novel and familiar stimuli occurs after a delay of an hour. This type of delay has not, to our knowledge, been reported in previous studies of single-trial learning, but given its importance of generalization in natural behaviour this type of learning may have wide relevance.