British ecologists have discovered that brightly colored European butterflies evade bird predators by using spots, stripes, and erratic flight patterns to create visual motion illusions, scrambling a predator’s ability to gauge the insect’s true speed and direction during attacks.
Naturalists have long wondered how brightly colored butterflies manage to evade the attention of birds that make a meal of other insects. Recent findings from British ecologists indicate that this relies on an optical trick generated by their flapping wings rather than pure fortune.
The investigation carried out by academics at the universities of Exeter and Essex determined that the markings present on numerous butterfly wings, paired with their erratic movement, disrupt a hunter’s perception of the creatures’ trajectory and velocity.
How Wing Deformations and Barber Pole Illusions Confuse Birds
George Hancock, of the University of Exeter’s Centre for Ecology and Conservation in Cornwall, noted that very few European butterflies are toxic or unpalatable. Because they are relatively easy to spot in flight, these insects need something special to stay out of harm’s way.
“The illusions interfere with the predator’s most basic visual targeting system and disrupt the final ballistic attack. As a result, birds and other predators often miss.”
George Hancock, University of Exeter’s Centre for Ecology and Conservation in Cornwall
Jolyon Troscianko, also of the University of Exeter, explained the mechanics behind the phenomenon. Butterfly wings deform as they fly, clapping together on the upstroke and peeling apart on the downstroke, so their stripes shift angle and point in different directions as they flap
he said, noting that combined with unpredictable flight paths, this patterning creates misleading motion illusions.
The experts compared the mechanism to the barber pole illusion
, where red and white stripes appear to travel upwards even though the pole itself is only spinning in place. To test this, Troscianko put a slow-motion video of a butterfly flying upwards through a bird-vision computer model, which wrongly indicated that the insect was moving downwards.
“First I checked this wasn’t a coding error, that I’d swapped up and down somehow, but then it dawned on me that this would be a perfect way to confuse attacking predators.”
Jolyon Troscianko, University of Exeter
Swallowtail Butterflies and the Evolutionary Roots of Motion Confusion
Looking across hundreds of types of European butterflies, the research team discovered that different patterns produce distinct effects. Swallowtail butterflies emerged as some of the best at using motion confusion.
Troscianko suggested that because swallowtails are thought to be closest to ancestral butterflies, this defensive mechanism might have originated right when butterflies evolved away from night-flying moths. These insects feature little tails trailing from their hindwings that help increase motion confusion. In the wild, swallowtails are frequently found with missing tails and beak-shaped holes cut from their wing edges, confirming that failed bird attacks hit less vulnerable areas.
Human Touchscreen Tests and Stark White Contrasts
To verify how these patterns affect attacks, the team used a high-speed camera to film taking-off butterflies and enlisted 100 human volunteers to act as predators trying to catch virtual butterflies on a touchscreen. The human participants struggled.
Beyond patterned wings, the study found that largely white butterflies also successfully dodge predators. Researchers believe that in flight, these white insects appear to move more quickly than their actual speed due to the stark contrast they form against their background, throwing off the timing of predatory strikes.
Broader Implications Across Nature and Published Findings
The discovery extends beyond butterflies. Researchers suspect motion confusion operates widely in the natural world, appearing in contexts ranging from the flapping wings of birds to the flicking tails of lizards and fish, opening an exciting new research avenue.
The complete findings are published in the journal Nature under the title Butterfly wing patterns in flight create powerful illusory motion cues.