We investigated the roles of signal reliability and environmental uncertainty in animal signal use. We developed a simple model that predicted when animals should switch between choosing the most common option (which we call environment tracking) and following a signal. The model predicts signal following when signal reliability exceeds environmental certainty. We tested this model experimentally using captive blue jays, Cyanocitta cristata. During each trial, the jays had to choose between two stimuli; one option was correct and led to food, and the other was incorrect and provided no reward. In addition, a third stimulus, the signal, provided information about which choice stimulus was correct. Using this procedure, we manipulated signal reliability (the probability that the signal matched the correct stimulus) and environmental uncertainty (the background probability that a given stimulus was correct) in a factorial experiment. Signal reliability and environmental uncertainty influenced signal use roughly as predicted. Jays used the signal when the signal was reliable and the environment was uncertain, and they ignored the signal when it was unreliable and the environment was predictable. Quantitatively, we observed a bias in favour of environment tracking. Jays sometimes ignored the signal when it could have helped them, and they ignored the signal in conditions where signal following and environment tracking produced equal payoffs. We discuss the implications of these findings.