Prey palatability influences predator learning and memory

Aposematic insect species are thought to gain protection from avian predators by advertising their unpalatability with bright or conspicuous coloration. However, individuals within a population may differ in their level of chemical defence. Since the speed and strength of avoidance learning are often assumed to be determined by the degree of defence, the presence of more palatable individuals in a population is thought to decrease the value of the warning coloration. Using chicks, Gallus gallus domesticus, as model predators and coloured crumbs flavoured with different concentrations of quinine sulphate solution for unpalatable prey, we tested these learning assumptions using a colour discrimination task. The mean level of prey defence was positively correlated with the speed of avoidance learning, and negatively correlated with the ease with which learned aversions were reversed. However, it was unclear how prey palatability influenced the strength of avoidance learning, since the number of unpalatable crumbs attacked at asymptote was not correlated with the mean level of prey defence, although the reluctance to attack an unpalatable crumb within a trial was positively correlated with prey defence. We also found that the proportion of the attacked unpalatable crumbs that were subsequently rejected was positively correlated with the mean level of prey defence. Our results show the importance of the level of prey defence in determining both the attack and ingestion rates of avian predators.