Pigeons can discriminate group mates from strangers using the concept of familiarity

It is widely accepted that group-living animals alter their behaviour towards a conspecific depending on whether it is known or unknown. To distinguish between group members and strangers in an efficient way, it would be adaptive for an animal to categorize conspecifics on the basis of familiarity. We investigated whether pigeons, Columba livia, are able to learn this categorization in a concept-like manner when trained with 2D images of conspecifics. Two of the six experimental pigeons were able correctly to classify test photographs of aviary mates which they had never seen as photographic stimuli before. Control animals, which had no real-life experience of the pictured conspecifics, all failed. This is a cognitively demanding task; not only are there no consistent visual features among group members, but photographic stimuli also present a much degraded version of the depicted pigeons. Further tests revealed that the familiarity concept could be transferred to nonconspecifics and that 24 h of exposure was enough for another pigeon to be classed as familiar. In sum, the findings show that pigeons (1) interpreted computer stimuli as representations of real entities, (2) possessed the ability to form a concept of familiarity and (3) were able to distinguish between social partners and strangers in nature.