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.

Overall, therefore, individuals should be selected to respond differently to different heterospecific alarm calls, depending at least partly on the reliability of information from the listener's perspective.

The response of 2 species to each other's alarm calls could be symmetric if each species is equally reliable to the other, or asymmetric if there is a difference in reliability, such as if 1 species is vulnerable to a subset of threats to the other.

2) “Discrimination.” Species can differ in the reliability with which they discriminate between stimuli that are threatening and nonthreatening.

For example, within mixed-species flocks in Sri Lanka orange-billed babblers ( ; Møller 1988 ) can use alarm calls as a way of scattering individuals of other species to steal food.

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.

If a large proportion of alarm calls by another species is given when there is no danger to the listener, it may be difficult to learn to associate the call with danger, even if it would be adaptive to do so.

In general, associative learning is likely to be faster if a novel stimulus is more predictably associated with the unconditioned stimulus ( Shettleworth 1998 ; Bouton 2007 ).

We used a playback experiment to test responses to heterospecific aerial alarm calls among 3 Australian passerines and natural observations to assess whether any differences in responses could be related to call reliability.

Superb fairy-wrens ( ) are pair breeders that are vulnerable to only a subset of threats and do not form mixed-species flocks with the other species.

Fairy-wrens and scrubwrens respond to playback of each other's aerial alarm calls ( Magrath et al.