Alarm calls of southern house wrens, Troglodytes aedon bonariae, convey information about the level of risk

Alarm calls are an antipredatory strategy widely used by animals. Some calls are functionally referential, giving specific information about the perceived threat. In other cases, the calls are less specific although they may also provide information about the level of threat or “urgency.” Here, we assess whether southern house wrens (Troglodytes aedon bonariae) provide information about the level of risk when they perceive a threat during nesting. We analysed the call acoustic structure and repetition rate of calls emitted by the breeding pair when we placed a model of a predator at different distances from the nest. The results showed that, although there were no structural differences in alarm calls among treatments, individuals increased the rate of alarm calling according to the distance of the predator model from the nest, reflecting a perceived level of threat. Playback experiments of alarm calls reproduced at different rates also showed that recruitment of conspecific and heterospecific individuals increased with the calling rate. These experiments showed that the rate of alarm calling in the southern house wren provides information about the perceived level of risk and that listeners respond accordingly. The functional meaning of alarm calls is a very important aspect in animal communication and in the analysis of prey defences against predators. From two field experiments, we show that house wrens vary their call rate in relation to the distance of a threat to the nest, and that this variation correlates with the response of conspecifics and heterospecifics. The results show that house wrens' alarm call rates provide reliable information about the level of risk involved.