Why are owl-like eyespots enough to trigger mobbing in some birds?

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A fluffy gray cat clings to the trunk of a tree, looking upward as it climbs to escape. — Avoidance behaviour Cat fleeing by upward climbing.

In the context of avoidance behaviour, owl-like eyespots are sufficient to trigger mobbing in many passerine birds because the eyes themselves serve as a key stimulus that instinctively signals the presence of a predator.1

Research indicates that these responses are largely innate rather than learned. Even birds reared in complete isolation will gather to mob when presented with man-made models featuring appropriate eyespots. This behavior is a form of warning behaviour, where birds advertise the presence of a predator that is potentially dangerous but not an immediate threat, such as an owl during the daytime. By focusing on the eyes, the birds identify a specific, recognizable feature of a known threat, prompting them to initiate mobbing to defend their territory or warn others.12