Jackdaws nest in holes, and suitable cavities are a limiting resource. Competition for them is intense, and an unattended hole is liable to be taken over. The species' unusual near-white iris stands out sharply against dark plumage in the gloom of a cavity entrance.
Tests placing photographs of jackdaw faces and isolated pale eyes at the entrances of occupied nest boxes found that approaching jackdaws were deterred by the eyes alone, and less so by the face without them. The signal appears to work specifically on conspecifics, which are the birds most likely to want that particular hole.
The finding fits the broader pattern of jackdaw social life: stable lifelong pairs, coordinated flock movements, and mobbing responses that are learned socially, so a bird that has seen a person handle a jackdaw will scold that person afterwards and others in the colony pick up the association. How far the eye signal also works on non-jackdaw competitors has not been resolved.