Avian predators change their foraging strategy on defended prey when undefended prey are hard to find

Visually hunting predators forage on prey possessing a range of different defence strategies. The two most commonly studied defence strategies are camouflage (in which palatable and undefended prey have colour patterns to minimize detection) and aposematism (in which toxic prey advertise their defences using conspicuous warning signals). Typically, these two defence strategies are studied in isolation, but when multiple prey species are eaten by the same predators, changes in the defence strategy of one species could affect the selection pressures acting on the defensive strategy of another. In this experiment, we tested this idea by investigating whether predators increased their foraging behaviour on prey that they knew to be toxic when undefended prey were better concealed. European starlings, Sturnus vulgaris, were allowed to forage freely for undefended (water-injected) and defended (quinine-injected and coated) mealworms, Tenebrio molitor, in bowls containing woodchip (prey type was signalled by bowl colour). Toxic prey were always covered with 30 ml of woodchip, while undefended prey were covered with either 30 ml or 150 ml of woodchip (all birds received both woodchip depths). Increasing the amount of woodchip covering the undefended prey made them harder to find, but did not affect birds' propensity to search for toxic prey. However, birds did attack and eat more toxic prey, but only when they received the 150 ml treatment before the 30 ml treatment. Our experiment shows that better concealed palatable prey can increase predation of toxic prey, with implications for the study of prey defences.