Alarming features: birds use specific acoustic properties to identify heterospecific alarm calls

Abstract

Vertebrates that eavesdrop on heterospecific alarm calls must distinguish alarms from sounds that can safely be ignored, but the mechanisms for identifying heterospecific alarm calls are poorly understood. While vertebrates learn to identify heterospecific alarms through experience, some can also respond to unfamiliar alarm calls that are acoustically similar to conspecific alarm calls. We used synthetic calls to test the role of specific acoustic properties in alarm call identification by superb fairy-wrens, Malurus cyaneus. Individuals fled more often in response to synthetic calls with peak frequencies closer to those of conspecific calls, even if other acoustic features were dissimilar to that of fairy-wren calls. Further, they then spent more time in cover following calls that had both peak frequencies and frequency modulation rates closer to natural fairy-wren means. Thus, fairy-wrens use similarity in specific acoustic properties to identify alarms and adjust a two-stage antipredator response. Our study reveals how birds respond to heterospecific alarm calls without experience, and, together with previous work using playback of natural calls, shows that both acoustic similarity and learning are important for interspecific eavesdropping. More generally, this study reconciles contrasting views on the importance of alarm signal structure and learning in recognition of heterospecific alarms.

Learning and signal copying facilitate communication among bird species

“…As a result, any feature of a signal that speeds up recognition should be favoured [30]. In some cases, heterospecific alarm responses occur without previous exposure [2,24,26]. For example, the responses of superb fairy-wrens (Malurus cyaneus) to the alarm calls of allopatric species depend on the similarity of their calls [2].…”

“…For example, despite divergence as a result of selection in other contexts, calls might include widely recognizable features [24], meaning calls are 'similar enough' across species for receivers to recognize them [25,26]. However, if divergence is too great for automatic recognition, repeated exposure could allow receivers to learn to associate the calls of other species with an appropriate alarm response [27,28].…”

Does signal deterioration compromise eavesdropping on other species' alarm calls?

“…We suggest that the solution to this puzzle is that conspecific receivers attend to specific acoustic features that are relatively invulnerable to degradation. In support of this idea, fairy-wrens attend primarily to peak frequency, the frequency at which amplitude is greatest, when deciding whether to flee (Fallow, Gardner, & Magrath, 2011;Fallow, Pitcher, & Magrath, 2013), and peak frequency is also resistant to degradation and little affected by reverberation (e.g. Wiley, 1991;Wiley & Richards, 1982).…”

“…For example, fairy-wrens always flee to normal New Holland honeyeater, Phylidonyris novaehollandae, aerial alarm calls, but rarely fled when the calls were played backwards, which altered call structure but not peak frequency or audibility (Magrath et al, 2009b). Similarly, synthetic calls with honeyeater call structure but fairy-wren peak frequency provoke about 45% of birds to flee, again showing that acoustic structure of heterospecific calls, and not just peak frequency, affects response (Fallow et al, 2013). Other bird species have greater difficulty discriminating heterospecific than conspecific calls (Benney & Braaten, 2000;Lohr et al, 2003).…”

“…Superb fairy-wrens fled to several unfamiliar heterospecific alarm calls that had peak frequencies similar to conspecifics, regardless of other acoustic features such as the presence of frequency modulation (Fallow et al, 2011). Similarly, with playbacks using synthetic alarm calls generated on computer, peak frequency prompts fleeing to cover often independent of other acoustic features (Fallow et al, 2013). Both studies also show that the time spent in cover after fleeing increases if calls are also similar to those of conspecifics in the rate of frequency modulation.…”