Acoustic discrimination of passerine anti-predator signals by Australian raptors

Abstract

Acoustic discrimination of anti-predator calls was examined in 11 species of Australian raptors, including 5 Falco species and 2 species of Elanus kites, by their responses to the playback of alarm and distress calls of Australian passerines. The present study investigated the ability of raptors to discriminate between alarm and distress calls that have different acoustic properties and are emitted in different behavioural contexts. The raptors were tested with broad-band calls (containing a wide range of frequencies) given as distress calls, mobbing calls and alarm calls to terrestrial predators, and with narrow-band calls (comprising a narrow range of frequencies) typically given as a response to flying predators. Raptor responses were categorised into three classes based on head orientation (or lack thereof) towards the sound source (i.e. one of 2 or 4 speakers positioned in the cage set-up); (1) ‘correct response’ – the raptor looked directly at the speaker; (2) ‘incorrect response’ – the raptor detected the sound, but oriented the head in a direction other than towards the sound source; (3) ‘no response’. All raptor species showed a higher percentage of correct responses (60–100%) for broad-frequency vocalisations and a lower percentage of correct responses (usually 0–40%) and more incorrect responses for narrow-band vocalisations. Further, all raptors showed a greater rate of overall responsiveness to broad-band alarm and distress calls than narrow-band calls, indicating a higher interest level in the former. The behavioural implications of acoustic discrimination by Australian raptors to different types of alarm call are discussed.