Vocal mimicry in the brown thornbill
Date
2013
Authors
Igic, Branislav
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Mimicry of other species' alarm calls is common among birds, but its function is often unknown. Although anti-predator functions for mimetic alarm calls have been suggested, these are yet to be tested. In this thesis I formally evaluated claims of vocal mimicry in the brown thornbill (Acanthiza pusilla) and tested if mimetic vocalisations function in preventing nest predation. Both male and female thornbills are capable of mimicking a range of different vocalisations, including heterospecific contact calls, songs, and alarm calls. Mimetic vocalizations also varied greatly in acoustic structure: both high and low frequency vocalisations were mimicked. Despite the diversity in the types of vocalisations thornbills can mimic, they predominantly mimicked heterospecific alarm calls when captured in mist-nets. Although these mimetic alarm calls were not identical to their corresponding model species' alarm calls, they did retain acoustic features of model alarm calls that are important for provoking immediate alarm responses by receivers. Brown thornbills mimicked heterospecific alarm calls more often in circumstances of danger, and especially when callers or their offspring were in extreme danger. These mimetic alarm calls communicated honest information about flying and stationary predators, but inaccurate information (presence of an aerial predator) when callers or their offspring were under attack. These findings suggest that the thornbill's mimetic alarm calls play different functions in different circumstances of danger. In nest defence, thornbills mimic heterospecific mobbing alarm calls (alarm calls given by heterospecifics towards terrestrial predators) if a predator is near their nest and mimic aerial alarm calls (alarm calls given by heterospecifics towards aerial predators) if the nest is under attack. Mimicking mobbing alarm calls towards predators near the nest potentially hides the caller's identity and may reduce risk of alarm calls revealing the nest's location to the predator. Mimicking aerial alarm calls when the nest is under attack potentially fools the predator that it is in danger, causing distraction and providing young with an opportunity to escape. A playback experiment on the thornbill's primary nest predator, the pied currawong (Strepera graculina), supported the hypothesis that mimetic aerial alarm calls cause distraction and provide nestlings with an opportunity to escape when attacked. Currawongs responded with typical alarm responses and delayed feeding following either the thornbill's non-mimetic aerial alarm alone or a combination of non-mimetic and mimetic alarms. Including mimetic alarms with non-mimetic alarms prolonged currawong distraction.
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