Discriminating between similar alarm calls of contrasting function

In a pioneering study of signal design, Marler (Marler 1955 Nature 176, 6–8; Marler 1957 Behav. 11, 13–37) argued that the contrasting acoustic design of hawk (seet) and mobbing alarm calls of European passerines reflected their contrasting function. Hawk alarms were high-frequency tones, warning con- specifics to flee but making localization difficult for predators, while mobbing calls were broadband and harsh, allowing easy localization and approach. Contrasting signal features are also consistent with signal detec- tion theory. Discriminating these calls quickly is critical for survival, because hawk alarms require immediate escape. These signals should there- fore be selected to be easy to discriminate, reducing the trade-off between immediate fleeing to hawk alarms and unnecessary fleeing to mobbing alarms. Despite these expectations, hawk and mobbing alarm calls of superb fairy-wrens, Malurus cyaneus, are surprisingly similar, raising the question of discriminability without conte...