Warning Signal Efficacy: Assessing the Effects of Color, Iridescence, and Time of Day in the Field

Warning coloration deters predators from attacking distasteful or toxic prey. Signal features that influence warning color effectiveness are not well understood, and in particular, we know very little about how effective short-wavelength and iridescent colors are as warning color elements in nature and how warning signal effectiveness changes throughout the day. We tested the effect of these factors on predation risk in nature using specimens of the distasteful pipevine swallowtail butterfly, Battus philenor. B. philenor adults display both iridescent blue and diffusely reflecting orange components in their warning signal. We painted B. philenor wings to create five different model types: all-black, only-iridescent-blue, only-orange, iridescent-blue-and-orange (intact signal), and matte-blue-and-orange. We placed 25 models in each of 14 replicate field sites for 72 h and checked for attacks three times each day. Model type affected the likelihood of attack; only-orange models were, the only model attacked significantly less than the all-black model. Iridescence did not enhance or decrease warning signal effectiveness in our experiment because matte-blue-and-orange models were attacked at the same rate as iridescent-blue-and-orange models. Time of day did not differentially affect model type. Video recordings of attacks revealed that insectivorous birds were responsible. The results of this experiment, when taken with previous work, indicate that the response to blue warning coloration is likely dependent on predator experience and context, but that iridescence per se does not affect warning signals in a natural context.