In one experiment, observed responses of the avian predators to the eyespots included increased vigilance, a delay in their return to the peacock butterfly, and the production of alarm calls associated with ground-based predators. These responses to the eyespot stimuli lend support to the eye mimicry hypothesis as they indicated that the avian predator sensed that the eyespots belonged to a potential enemy. When faced with avian predators like the blue tit, the peacock butterfly makes a hissing noise as well as threateningly displaying its eyespots. However, it is the eyespots that protect the butterfly the most; peacock butterflies that have had their sound production capability removed still defend themselves extremely well against avian predators if their eyespots are present.