A mutual understanding? Interspecific responses by birds to each other's aerial alarm calls

Individuals are likely to benefit from responding to the alarm signals of other species with similar predators, and mutual interspecific responses to aerial (hawk) alarms are thought to be common in birds, in part because similarity in alarm call structure among species might facilitate detection or interpretation. However, there has been no test of whether interspecific responses to aerial alarm calls can involve mutual responses between species and only incomplete tests of the response of any species to such heterospecific alarms. We describe the aerial alarm calls of white-browed scrubwrens (Sericornis frontalis) and superb fairy-wrens (Malurus cyaneus) and use a playback experiment to test for mutual responses to each other's aerial alarm calls. The 2 species occur in similar habitats and can co-occur in mixed-species flocks during the nonbreeding season. The aerial alarm calls of both species are high pitched (≥7 kHz) and rapidly frequency-modulated calls but are distinct in frequency measures and only the scrubwren's call had 2 parallel sounds. Both species fled to cover after playback of either their own or the other species' alarm calls but never to control sounds. The response to either species' alarm was almost invariant in both species in an experiment at high natural amplitude, but there was a slightly lower response to heterospecific compared with conspecific alarms when playbacks were at the mean natural amplitude. Our results demonstrate, after at least 50 years of interest in the subject, that there can be mutual responses to aerial alarm calls between species.