Birds learn to avoid aposematic prey by using the appearance of host plants

The conspicuous warning signal of aposematic animals is learned by their predators, and the resulting avoidance benefits both parties.^1-4 Given evidence that birds can distinguish the profitability of prey from the environmental context in which they appear,⁵ aposematic insects' host plants might also provide an important cue to foraging predators.⁶ The aposematic cinnabar moth (Tyria jacobaeae) larva is a specialist on its ragwort (Senecio spp.) host plant,⁷ presenting a consistent environment with which it could be reliably associated. Additionally, ragwort's defensive toxins prevent non-specialist, profitable insects from feeding on it.⁸ Thus, avian predators may recognize cues from ragwort, most likely its conspicuous yellow flowers,^9,10 and use this information to avoid cinnabars. To test this hypothesis, we exposed artificial cinnabar and non-signaling "caterpillar" targets to wild avian predation by presenting them on ragwort and non-toxic host plants. We also manipulated the presence or absence of ragwort flowers on hosts. In doing so, we show that both targets are better protected on the cinnabar's natural ragwort host and that birds use ragwort's distinctive yellow flowers as the cue to avoidance. Additionally, we found that naive predators do not make prey host foraging distinctions, indicating that this avoidance behavior is learned through experience. Our findings are among the first to suggest that a host plant's features act as an extended phenotype that signals the toxicity of the prey that live on it. This prey-host relationship may facilitate the initial evolution of toxicity in non-signaling prey, but also inhibit the evolution of aposematic signals themselves. VIDEO ABSTRACT.