Distastefulness as an antipredator defence strategy

Many prey species have evolved bitter-tasting toxins that effectively protect them from potential predators. While predators can learn to associate the taste of defended prey with the noxious effects of the toxins, it is unclear whether bitter tastes also function as deterrents. We explicitly tested the effectiveness of a nontoxic distasteful chemical as an antipredator defence. We gave four groups of European starlings, Sturnus vulgaris, a sequential colour discrimination task, where one colour signalled undefended mealworms, Tenebrio molitor, and the other signalled defended mealworms that had either been injected with or coated with either a high or a low concentration of Bitrex solution (a nontoxic bitter-tasting solution). Birds ate all of the prey presented in this way, but performed disgust responses after eating prey coated in Bitrex solution. Birds were then given a series of trials in which they received defended and undefended prey items simultaneously. Birds that received prey injected with Bitrex attacked similar numbers of defended and undefended prey, whereas birds that received prey coated in Bitrex ate significantly fewer defended than undefended prey. Birds given prey coated in a high concentration of Bitrex showed a stronger preference for undefended prey than birds given prey coated in a low concentration of Bitrex. Our experiment demonstrates that a nontoxic distasteful chemical can protect insect prey from predation, but only under very specific conditions.