Predator avoidance learning of prey with secreted or stored defences and the evolution of insect defences

Chemically defended insects sometimes advertise their unpalatability to foraging avian predators by using warning displays that predators learn to avoid. One important aspect of many warning displays is the secretion of some or all of the chemical defence upon attack. In this experiment, we used European starlings, Sturnus vulgaris, foraging on mealworms, Tenebrio moloitor, to investigate how secreting defence chemicals influences the way in which predators assess palatability and learn to avoid unpalatable prey. We gave two groups of starlings a sequential colour discrimination task, where one colour signalled palatable mealworms and the other signalled mealworms that had been either injected with or coated with quinine solution (designed to simulate prey that stored and secreted their chemical defences, respectively). Birds in both groups readily acquired the colour discrimination, but differed in how they avoided the unpalatable prey. Birds presented with mealworms with secreted defences learned to avoid unpalatable prey on the basis of their colour signals more quickly than birds presented with mealworms with stored defences. However, they also attacked more unpalatable individuals at asymptote, perhaps because they learned to wipe the quinine from the surface of the mealworms. This suggests that there may be an optimum strategy for secreting chemicals that could explain why many unpalatable insects secrete only a proportion of their defence chemicals.