Ravens judge competitors through experience with play caching.

Complex social behavior builds on the mutual judgment of individuals as cooperation partners and competitors [1]. Play can be used for assessing the others' dispositions in humans and nonhuman mammals [2], whereas little is known about birds. Recently, food-caching corvids have been found to rival primates in their ability to judge the behaviors and intentions of others in competition for hidden food [3]. Here, we show that ravens Corvus corax quickly learn to assess the competitive strategies of unfamiliar individuals through interactions with them over caches with inedible items and subsequently apply this knowledge when caching food. We confronted birds with two human experimenters who acted differently when birds cached plastic items: the pilferer stole the cached objects, whereas the onlooker did not. Birds responded to the actions of both experimenters with changing the location of their next object caches, either away from or toward the humans, as if they were testing their pilfering dispositions. In contrast, ravens instantly modified their caching behavior with food, preventing only the competitive human from finding the caches. Playful object caching in a social setting could thus aid ravens in evaluating others' pilfering skills.