Defended (distasteful or toxic) prey are often characterized by conspicuous coloration and this phenomenon is called "aposematism". The main advantage of aposematism is that it promotes faster learning by predators to avoid the prey. Some defended prey species use a different strategy; they remain cryptic in the normal state, but display conspicuous aposematic signal (which is normally hidden) in response to a predator's approach/attack. This anti-predator strategy of a defended prey has not been well studied yet although it can theoretically give the benefits of both camouflage and aposematism. Here, we investigated the effectiveness of this ‘hidden-aposematic signal’ as a warning signal. Using wild tits (Parus minor) as predator and novel artificial prey models (which mimics wings of insects), we tested whether hidden conspicuous signal of a defended prey enhances the avoidance learning rate of predators and how does it compare with the typical conspicuous/non-conspicuous signal. We found that hidden conspicuous signal indeed enhances the avoidance learning rate of predators in comparison with the non-conspicuous signal. However the overall learning rate by predators to avoid the defended prey was slower than for the normal conspicuous signal. Our results suggest that the prey with hidden-aposematic signals could enjoy both the benefits camouflage and the benefits of aposematism that are however lower than benefits from a typical aposematic signal. We, for the first time, highlight the functional aspect of a unique, but yet largely ignored, defensive coloration of prey.