At rest the peacock is nearly black beneath and closely resembles a dead leaf. When disturbed it opens its wings suddenly, exposing four large eyespots against red. The same wing movement produces a broadband sound: friction between the forewing and hindwing bases generates a click train perceived as hissing, with substantial ultrasonic content.
Controlled experiments have separated the two signals. Against birds such as blue tits, painting out the eyespots sharply increased attacks, while muting the sound made little difference. Against small nocturnal rodents in the dark, where eyespots cannot be seen, the sound-producing individuals survived encounters better than silenced ones. The butterfly therefore carries two defences aimed at different predator classes and different lighting conditions.
The ultrasonic component has prompted the suggestion that the sound targets bat hearing, since overwintering peacocks are vulnerable in caves and cellars. Evidence for a specifically anti-bat function is suggestive rather than conclusive.