The 'seet' danger call: An active nest warning in superb fairy-wrens

Alarm calls are a common, widely studied antipredator behaviour, with species producing different call types that can have distinct functions. Here, we describe a new type of alarm call in the superb fairy-wren (Malurus cyaneus), the 'seet' danger call. We (i) describe the acoustic characteristics and context of the danger call (in comparison with two other alarm calls); (ii) test its occurrence in relation to predator proximity to nest and nesting stage; and (iii) test the response of birds to experimental broadcast of aerial versus danger calls, given their similar acoustic structure. The danger call is a high-frequency, down-sweep call. Danger calls were produced in all nesting stages, with more calls given when the human threat was closer to the nest. Most danger calls were produced during the feeding phase, and the fewest were produced after fledging. Groups produced more danger calls when responding to experimental broadcasts of danger versus aerial calls. We propose that the 'seet' danger call functions as a warning to conspecifics of a threat approaching immobile or moderately mobile and, hence, vulnerable individuals. The occurrence of a nest-based alarm call that covaries with nest content raises the question of which cues adults use to assess offspring vulnerability.