Data from: Predator mimicry, not conspicuousness, explains the efficacy of butterfly eyespots

Large conspicuous eyespots on butterfly wings have been shown to deter
predators. This has been traditionally explained by mimicry of vertebrate
eyes, but recently the classic eye-mimicry hypothesis has been challenged.
It is proposed that the conspicuousness of the eyespot, not mimicry, is
what causes aversion due to sensory biases, neophobia or sensory
overloads. We conducted an experiment to directly test whether the
eye-mimicry or the conspicuousness hypothesis better explain eyespot
efficacy. We used great tits (Parus major) as model predator, and tested
their reaction towards animated images on a computer display. Birds were
tested against images of butterflies without eyespots, with
natural-looking eyespots, and manipulated spots with the same contrast but
reduced resemblance to an eye, as well as images of predators (owls) with
and without eyes. We found that mimetic eyespots were as effective as true
eyes of owls and more efficient in eliciting an aversive response than
modified, less mimetic but equally contrasting eyespots. We conclude that
the eye-mimicry hypothesis explains our results better than the
conspicuousness hypothesis and is thus likely to be an important mechanism
behind the evolution of butterfly eyespots.