Repetitive calls in Southern House Wrens: information encoded in the number and rate of alarm calls

Abstract

Birds often use acoustic alarm signals to alert co- and heterospecifics about nearby threats. Callers can use single calls or longer sequences of repetitive calls, which can produce a sustained effect on the behavior of the signal targets. The rate at which the alarm calls are given can also reveal the ‘urgency’ or level of risk perceived by the caller. In Southern House Wrens, Troglodytes musculus, breeding individuals give repetitive alarm signals when a threat is detected near the nest. To understand the information that encodes the number and calling rate of alarm signals produced by Southern House Wrens, we tested the individuals’ responses to experimentally manipulated signals. We broadcast alarm calls that differed in calling rate and length (i.e., the number of calling notes) and recorded the response probability, response latency, minimum approach distance to the speaker, and the time spent near the speaker by conspecifics. Wrens responded more frequently and quickly to alarm calls repeated at higher rates. Furthermore, individuals spent more time near the speaker when we played back calls at a high rate and when we played back longer sequences of calls. However, individuals also got closer to the speaker when we broadcast long call sequences at a low rate. We propose that the repetition of calls could reveal the persistence of the stimulus, whereas the calling rate could encode information about the risk level detected. These findings contribute to the understanding of how receivers extract the information from conspecific alarm signals.