Aposematic animals use anti-predator defence mechanisms such as distastefulness coupled with distinctive odours, sounds, or colour visual signals to predation from domestic chicks we show that the protective value of such visual warning displays is enhanced by increasing size of the signal pattern elements and decreased by pattern asymmetry. These results provide the first experimental evidence that predation may select for individual symmetry of visual warning displays, and concur with earlier demonstrations that asymmetric signals are more difficult to detect, learn, and remember, compared to symmetric signals. Collectively, our findings suggest that prey species possessing warning coloration should be subjected to selection for large and symmetric pattern elements.