The African hornbill is one of the few species of animals that can understand the language of another. More specifically, the hornbill is conversational in Diana monkey.
A study at the University of St. Andrews found that African hornbills are able to understand the warnings of the chatty Diana monkey. The monkey gives distinct warning calls to other monkeys based on possible predators it sees. The warning for a leopard is different from the warning for a crowned eagle, for example. Researchers noticed that the hornbills would scatter upon hearing the warning for the eagle, but not for the leopard, because the leopard isn't much of a threat to an animal that can fly. Researchers tested this by playing back recordings of both of those calls, and sure enough, the hornbills only responded to the eagle warning.