An avian eavesdropping network: Alarm signal reliability and heterospecific response

Alarm calls potentially provide information about predators to heterospecifics, but little is known about patterns of eavesdropping among species. Many cases of eavesdropping in birds and mammals involve social species in mixed-species groups, but this is not always true and the reliability of information may also be critical. We used a playback experiment and observations of natural alarm calling to test for understanding of aerial "hawk" alarms among 3 species of passerine and assess call reliability. Superb fairy-wrens and white-browed scrubwrens are ecologically similar and can share mixed-species flocks, whereas New Holland honeyeaters are ecologically distinct and do not flock with the other species. Fairy-wrens and scrubwrens fled to cover to each other's alarm calls, but they also both fled to honeyeater alarms. Honeyeaters fled to scrubwren but usually not fairy-wren alarms. The pattern of heterospecific responses appears related to call reliability from each species' perspective. Honeyeaters called only to predators of all 3 species and so provided reliable information to all. From a honeyeater's perspective, fairy-wrens were least reliable, as they gave 52% of their calls to nonpredators, whereas scrubwrens gave only 18% to nonpredators. However, from a scrubwren's perspective, fairy-wrens were largely reliable because most calls to nonpredators were to red wattlebirds, which pose a physical threat to fairy-wrens and scrubwrens but not honeyeaters. We conclude that there can be mutual responses to alarm calls between ecologically distinct species, that responses can be symmetrical or asymmetrical between species, and that call reliability appears to affect response.