Communicating about danger: urgency alarm calling in a bird

Vertebrate flee alarm calls can provide information about the type of predator, and some mammalian alarm calls also appear to communicate the degree of danger and therefore urgency of escape. However, because predators are usually rare, it has proved difficult to obtain observations differing only in the degree of danger, or to record sufficient naturally provoked alarm calls for fully replicated playback experiments. In this study, we took advantage of a system in which the major aerial predator was common, allowing repeated, matched observations of natural interactions between predator and prey, combined with a fully replicated playback experiment. We found that the aerial trill alarm call of the white-browed scrubwren, Sericornis frontalis, varied according to the distance from the suddenly appearing predator: the closer the predator, the greater the number of elements in the call and the higher their minimum frequency (pitch). Playback experiments showed that multi-element alarm calls prompted a more urgent response, including immediate fleeing to cover. Furthermore, the response was graded, such that an alarm with more elements provoked a more urgent response. Our study therefore isolated the effect of predator distance on alarm call design, and showed that individuals respond appropriately to calls in the absence of any other cues. To our knowledge, this is the most explicit demonstration that avian flee alarm calls can convey urgency.