The responses of black-casqued hornbills to predator vocalisations and primate alarm calls

Black-casqued hornbills (Ceratogymna atrata) forage in small flocks in the tropical forests of West Africa, often in the vicinity of primate groups, including Diana and Campbell's monkeys (Cercopithecus diana, C. campbelli). Previous work has shown that these monkey species produce acoustically distinct alarm calls to crowned eagles (Stephanoaetus coronatus) and leopards (Panthera pardus), two of their main predators. Black-casqued hornbills are highly vulnerable to crowned eagles, but not leopards, suggesting that individuals may respond differently to these two predators. We analysed the vocal response of these birds to field playbacks conducted on different monkey species in Tai National Park, Ivory Coast. We tested six stimuli, three of which related to the presence of a crowned eagle (eagle shrieks, Diana and Campbell's eagle alarm calls) and three to the presence of a leopard (leopard growls, Diana and Campbell's leopard alarm calls). Results showed that hornbills consistently distinguished between eagle- and leopard-related stimuli, Suggesting that birds attended to the predator class associated with the various stimuli. Second, within eagle-related stimuli, hornbills responded more strongly to the actual predator vocalizations than the associated alarm calls. One interpretation of these data is that birds were sensitive to the precision of information concerning the location of the eagle. We discuss these results in light of previous data on hornbill behaviour and cognitive capacities.