Antipredator behavior in blackbirds: habituation complements risk allocation

Several studies showed that animals allow closer approaches (measured through flight initiation distances, FIDs) by potential predators (e.g., humans) in high-predator density areas, which has been explained by habituation effects. We assessed whether this pattern could be produced by not only habituation but also risk allocation by simulating attacks on blackbirds Turdus merula by both usual (pedestrians) and novel (radio-controlled vehicle) potential predators in parks with different levels of human visitation. Individual blackbirds from parks with higher pedestrian rates showed lower FID than individuals from parks with lower pedestrian rates, in response to both usual and novel approaches. Blackbirds adjusted their antipredator behavior to the specific level of pedestrian rate encountered every morning and evening in each park, with higher FID in the period with lower pedestrian rate. Similar responses to usual and novel potential predators among parks and daily variation in antipredator behavior support the risk allocation hypothesis and could not be explained by habituation. However, the rate at which FID was reduced in individuals from low-visited parks to high-visited parks was greater for pedestrian attacks than for novel potential predator attacks, suggesting that habituation is also present in our system and complements the effects of risk allocation. Our results have applied implications: the reduction in FID with increasing human visitation in natural areas is usually attributed to habituation; however, we propose that risk allocation can also reduce antipredator behavior effort to survive in habitats with high levels of recreational activities at the expense of potential physiological costs.