Rat-wise robins quickly lose fear of rats when introduced to a rat-free island

Differences between continental and island species in their ability to recognize mammalian predators are well documented, but how quickly acquired predator recognition behaviour declines or is lost when animals are translocated to sites without mammals has not been fully investigated. We compared predator recognition in a ‘mainland’ population of Stewart Island robins, Petroica australis rakiura, where rats have been present since the 1600s, and in a reintroduced island population where rats were recently eradicated and the resident robins have experienced a rat-free environment for about one generation. We found that the rat-free island population showed little fear or recognition of a model rat and were less agitated and more likely to approach and consume food in front of the model rat relative to robins on the mainland. The results of our study suggest that endemic species that acquire the ability to recognize mammalian predators lose this ability relatively quickly when translocated to islands where mammals are absent. These results raise questions about the costs of maintaining mammalian recognition behaviour in mammal-free environments, even when avian predators are present. They also raise concerns about whether island sanctuaries are appropriate sources for harvesting for reintroductions back to the rat-infested mainland.