Are animals susceptible to optical illusions? What fish and birds can teach us about perception

The research question is: Do other animals also “fall for” the same tricks? If a small fish or a chicken is exposed to the illusion, what does this tell us about the way they see and interpret their environment?

Have you ever looked at two circles of equal size and been convinced that one was bigger than the other? If so, your eyes have been fooled by the Ebbinghaus illusion—a classic example of how context shapes what we see. Place a circle between smaller circles and it appears larger; place it between larger circles and it appears smaller. The illusion intrigues psychologists because it reveals that perception is not a reflection of the external world, but a deliberate construction of the brain.

This led to our research question: Do other animals also “fall for” the same tricks? If a small fish or a chicken is exposed to the illusion, what does this tell us about the way they see and interpret their environment?

Illusions are not just for fun. They are powerful tools for understanding how the brain organizes sensory information. When perception is “wrong,” the shortcuts and strategies the brain uses to organize complex environments are revealed.

In humans, the Ebbinghaus illusion is associated with global processing: the tendency to interpret a scene as a whole without focusing on details. But not all animals live in the same sensory world as we do. By examining illusions across species, we can ask whether shared patterns indicate deep evolutionary roots, or whether differences indicate adaptations to particular ecological niches. For example, global processing may have evolved in species that are required to quickly integrate complex scenes—such as detecting predators or estimating group size—while local processing is favored in species that rely on precise identification of items, such as foraging for seeds or prey in a crowded environment.

Fish vs. Birds: Two Worlds of Vision

To examine this, we turned to two very different species: the gobies (Poecilia reticulata) and the ring-dove (Streptopelia risoria).

Gobies live in shallow tropical streams, flooded with flickering light, dense vegetation and unpredictable predators. Their survival depends on quick decisions: choosing a mate, joining groups and fleeing danger. In such a busy world, making a quick judgment of relative size can be critical.

In contrast, ring-doves are terrestrial seed eaters. They spend a lot of time pecking at small seeds scattered on the ground. Accuracy and attention to fine details may be more important than analyzing the whole picture. In addition, their binocular vision allows them to judge distances and sizes in a very different context.

By placing the species side by side, we asked: Would the same illusion deceive both a fish darting through the water and a bird searching on the ground?

Circles of deception

In the experiments, we used food as the central “circle.” The bodies were placed with food flakes within arrays of small or large circles around them. The pigeons were presented with millet seeds in similar arrangements.

The results were stark: The pigeons consistently “fell” into the illusion. When the food was surrounded by small circles, they preferred it more often—as if it were actually larger. Their perception closely mirrored that of humans. The ring-doves, however, presented a different picture. At the group level, there was no consistent sensitivity to the illusion. Some individuals behaved like humans, others the opposite, and many were barely affected. This variability suggests that the pigeons rely on different perceptual strategies; more local, detail-focused, and less dependent on environmental context.

why is it important?

On the surface, this is just an amusing visual trick. But the findings touch on deep questions in evolutionary biology and comparative cognition: perception is not about accuracy per se, but about what is useful in the environmental context. For organisms, rapid integration of the whole may help navigate complex visual streams, identify large mates, or estimate relative sizes within a flock. For pigeons, which aim to forage for seeds against a “messy” background, focusing on absolute size and local details may be more useful.

The study also suggests that intra-species variation can be as important as differences between species. The pigeons' varied responses suggest that life experience or innate biases strongly shape the interpretation of illusions. Just as in humans – some are easily influenced by illusions and others are hardly affected – so too is perception not uniform in animals.

A window to other minds

Comparing distant species like fish and birds reveals a fascinating diversity of perceptual worlds. The Ebbinghaus illusion is just one of the tools that researchers use to map these worlds, but it highlights a fundamental point: what we see is not always what is really there.

For humans, it’s a reminder of the brain’s creative shortcuts. For animals, it’s evidence that ecological pressures shape perception to suit each species’ lifestyles. And for science, it’s a window into the evolutionary origins of cognition itself. Examining illusions across species helps us understand not only how animals see, but also how perception evolves to meet the challenges of life on Earth.

for the scientific article

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