Ravens, Corvus corax, differentiate between knowledgeable and ignorant competitors.

Human social behaviour is influenced by attributing mental states to others. It is debated whether and to what extent such skills might occur in non-human animals. We here test for the possibility of ravens attributing knowledge about the location of food to potential competitors. In our experiments, we capitalize on the mutually antagonistic interactions that occur in these birds between those individuals that store food versus those that try to pilfer these caches. Since ravens' pilfer success depends on memory of observed caches, we manipulated the view of birds at caching, thereby designing competitors who were either knowledgeable or ignorant of cache location and then tested the responses of both storers and pilferers to those competitors at recovery. We show that ravens modify their cache protection and pilfer tactics not simply in response to the immediate behaviour of competitors, but also in relation to whether or not they previously had the opportunity of observing caching. Our results suggest that the birds not only recall whom they had seen during caching, but also know that obstacles can obstruct the view of others and that this affects pilfering.