Population Variation in Mobbing Ospreys (Pandion haliaetus) by American Crows (Corvus brachyrhynchos)

American Crows (Corvus brachyrhynchos) aggressively mob a variety of natural predators and learn to recognize unique threats. Because mobbing is a costly and risky behavior, we hypothesized that crows would selectively ignore benign heterospecifics that look similar to predators, perhaps even learning to do so. Through a series of natural observations and experiments we found that American Crows were more likely to mob Red-tailed Hawks (Buteo jamaciensis) and Bald Eagles (Haliaeetus leucocephalus) than Ospreys (Pandeon haliaetus). Mobbing intensity was higher to a taxidermic mount of a Red-tailed Hawk than to a mount of an Osprey, indicating that mobbing increases with the risk posed by the predator. However, we also found that Ospreys were more likely to be mobbed in locations where they rarely occur, suggesting that crow populations that frequently encounter Ospreys habituate to this benign raptor. The extensive distribution of Ospreys and resulting co-occurrence with many mobbing species suggests our findings may have wide application.