A bird's alarm calls do not always come out of its beak

Nice primaries, dahling CHARLES DARWIN was fascinated by bird communication. In "The Descent of Man and Selection in Relation to Sex" he devoted equal space to both the sorts of sounds that emerge from birds' beaks and the more percussive noises that they make with other parts of their bodies, such as their feet and feathers. He speculated that both types of sounds were important for sending signals to others, but was unsure if this was true. In the years that have passed since his death, ornith...