Signals of profitability? Food colour preferences in migrating juvenile blackcaps differ for fruits and insects

Red is a common colour signal in both aposematic warning displays, and in fruit displays. One common feature is that red is conspicuous against the natural background of the prey and fruits. However, there is a potential conflict between fruits and aposematic prey in how a bird predator should react to red colours, where fruits aim to attract birds and aposematic insects aim to ward off, often the same bird individuals. Here we investigate possible differences in red/green colour preferences of frugivorous, wild-caught, young blackcaps (Sylvia atricapilla), when food is either a fruit or an insect. Birds in two groups were presented with a series of pairs of food items that had been artificially painted red and green, in the order of (I) fruits, crickets and maggots, or (II) crickets, fruits, and maggots. Birds first presented with crickets or fruits differed in first attacks directed at the two colours: They showed no colour preference between fruits, but showed a clear preference for green over red crickets. Also, birds in both experimental groups clearly preferred green to red maggots. These results provide evidence that wild, frugivorous birds are able to differentiate between prey types, and show different colour preferences depending on whether food is insect or fruit. We conclude that blackcaps show an attack bias against red insects, and that one important function of the signal in insects, is to inhibit attack after discovery. However, the lack of preference for red fruits suggests other functions to red fruit displays, such as facilitating discovery per se, rather than directly stimulating attack after discovery.