Seeing red? Colour biases of foraging birds are context dependent.

Colours are commonly used as visual cues when measuring animals' cognitive abilities. However, animals can have innate biases towards certain colours that depend on ecological and evolutionary contexts, therefore potentially influencing their performance in experiments. For example, when foraging, the colour red can advertise profitable fruits or act as a warning signal about chemically defended prey, and an individual's propensity to take food of that colour may depend on experience, age or physical condition. Here, we investigate how these contexts influence blue tits' (Cyanistes caeruleus) and great tits' (Parus major) responses to red-coloured almond flakes. We found that juvenile birds preferred red both when it was presented simultaneously with green, and when it was presented with three alternative colours (orange, purple, green). Adult birds, however, only preferred red after a positive experience with the colour, or when it was presented with the three alternative colours. We then tested whether colour influenced avoidance learning about food unpalatability. Despite the prediction that red is a more salient warning signal than green, we found only weak evidence that birds discriminated red unpalatable almonds from a green palatable alternative more quickly than when the colours were reversed. Our results suggest that biases towards red food may depend on birds' age and previous experience, and this might influence their performance in experiments that use red stimuli. Considering the ecological relevance of colours is, therefore, important when designing experiments that involve colour cues.