Iridescent blue and orange components contribute to the recognition of a multicomponent warning signal

Warning colouration functions to deter predators from attacking unprofitable (e.g., unpalatable or toxic) prey items. While warning colours are often characterized by long-wavelength components (e.g., orange, red or yellow), many distasteful animals across the animal kingdom display orange or red colour patches adjacent to or within a field of short-wavelength colouration such as blue (e.g., strawberry poison dart frogs, pipevine swallowtail and pyjama nudibranch), which yields a multicomponent visual warning signal. Here we show that, in such signals, blue and orange patches can function as redundant signal components; avian predators trained not to attack the intact blue and orange colouration of the pipevine swallowtail (Battus philenor) recognised the butterflies as distasteful even when the blue and orange were presented individually. Our results demonstrate that blue colouration and potential multiple, unimodal, signal components should be considered in research on visual warning signals, including in well-studied animals, such as dendrobatid frogs and swallowtail butterflies.