Effects of directionality, signal intensity, and short-wavelength components on iridescent warning signal efficacy

Many bright colors function as visual signals and are produced by structural mechanisms that result in their being iridescent. Iridescent colors differ from other types of animal coloration in that they vary in both hue and intensity, as the angle of viewing and illumination change relative to the color surface. Iridescent colors can serve as warning signals by deterring predation on distasteful animals. Due to their directional and mirror-like reflection, iridescent signals have the potential to (1) vary in appearance with each approach; (2) create a flashing signal; (3) under the right conditions, be more intense than diffusely reflecting signals; (4) display different hues with different positioning of the sun, predator, and prey; and (5) display angle-dependent camouflage, which could all influence warning signal effectiveness. Here, we examine how signal intensity, short-wavelength hue, and variation in appearance affect the response of domestic chickens (Gallus gallus domesticus) to warning signals. Variation did not affect their response. Higher intensity signals were more effective in terms of predator avoidance than were lower intensity, and blue-violet signals were more effective than were blue and blue-green. This could influence the effectiveness of an iridescent warning signal in nature if the prey displays more intense, blue-violet coloration. With the properties we tested, we found no cost to having warning colors that are iridescent and that there may be benefits in the ability to change hue and display a more intense signal. We suggest future research to examine the effects of iridescent flashing and angle-dependent camouflage on warning signal effectiveness. SIGNIFICANCE STATEMENT: Iridescent colors have only been recently shown to function as warning signals, but we know very little about how they function to deter predation. Here, we show that birds are more likely to change their behavior in response to higher intensity and blue-violet signals compared to low intensity and blue and blue-green signals. The intensity results are consistent with previous experiments, but the role of short-wavelength hue on warning signal effectiveness has never been tested before. Variation due to the shifting appearance of an iridescent signal did not influence the response to the warning signal, consistent with studies in long-wavelength signals. We found no cost to displaying an iridescent signal and potentially an adaptive benefit as iridescent warning signals have the potential to be brighter and display a different hue based on the arrangement of light source, signaler, and receiver.