Blue jays were used in laboratory experiments to determine what visual characteristics of Catocala hindwings are important in eliciting and maintaining startle reactions in avian predators. Hand-reared blue jays, inexperienced with warningly coloured, inedible prey, took significantly longer to touch novel colours that possessed bold, black bands than to touch novel, unbanded colours when these discs covered a food reward. Thus, being conspicuous (as opposed to simply being novel) appears to enhance startle reactions. Bold patterns are commonly found on aposematic prey and Catocala hindwings. These same hand-reared birds also tended to take longer to touch novel coloured discs of yellow and red hues as opposed to discs of blue, green, or purple hues. The colours that produced the greatest hesitation are similar to the colours found on Catocala hindwings. Red-yellow colours are also characteristic of aposematic prey. Wild-caught birds took longer to habituate to startling stimuli when presented with several different startling disc types during the habituation process. Thus, predator pressure may explain why several different Catocala species with different hindwing colours occur sympatrically.