Colour biases are more than a question of taste

Unpalatable insects often advertise their defences to predators using conspicuous colours, such as red and yellow. Perhaps not surprisingly, birds show unlearned biases against warningly coloured food. These biases are particularly evident when other components of insect warning displays, such as sounds and odours, are also present. Quinine, a bitter-tasting toxic chemical, can also enhance unlearned biases against red and yellow prey in naïve birds. However, whether this behaviour is performed specifically in response to quinine (which is chemically similar to many insect toxins) or can be elicited by other bitter-tasting chemicals is not known. The aim of our experiments was twofold. First, we investigated whether Bitrex, a bitter-tasting, nontoxic, man-made chemical, elicits colour biases similar to those elicited by quinine. Second, since avoidance learning can be affected by the number of bitter chemicals present in a prey population, we investigated whether the presence of both quinine and Bitrex enhanced unlearned biases against red crumbs compared to either chemical alone. We found that only quinine elicited attack biases against red prey, and there was no evidence that quinine and Bitrex in combination produced a stronger bias against red crumbs than quinine alone. These results indicate that colour biases incited by defence chemicals are chemical specific and may occur only in response to natural or toxic chemicals.