Sunday, June 7, 2015

[Ornithology • 2015] Crying Wolf to A Predator: Deceptive Vocal Mimicry by A Bird Protecting Young

Figure 1. Predator–prey relationships between species, and aerial alarm vocalisations used in experiments.
(a) From top to bottom: brown goshawk Accipiter fasciatus (top predator), pied currawong Strepera graculina (nest predator), brown thornbill Acanthiza pusilla (mimic), New Holland honeyeater Phylidonyris novaehollandiae (harmless species mimicked); arrows indicate direction of predator–prey relationship. (b) Spectrograms of brown thornbill non-mimetic aerial alarms, New Holland honeyeater aerial alarms and corresponding mimicry by brown thornbills.
Photo credits: brown goshawk, Geoffrey Dabb; rest, Steve Igic. (DOI: 10.1098/rspb.2015.0798)


Animals often mimic dangerous or toxic species to deter predators; however, mimicry of such species may not always be possible and mimicry of benign species seems unlikely to confer anti-predator benefits. We reveal a system in which a bird mimics the alarm calls of harmless species to fool a predator 40 times its size and protect its offspring against attack. Our experiments revealed that brown thornbills (Acanthiza pusilla) mimic a chorus of other species' aerial alarm calls, a cue of an Accipiter hawk in flight, when predators attack their nest. The absence of any flying predators in this context implies that these alarms convey deceptive information about the type of danger present. Experiments on the primary nest predators of thornbills, pied currawongs (Strepera graculina), revealed that the predators treat these alarms as if they themselves are threatened by flying hawks, either by scanning the sky for danger or fleeing, confirming a deceptive function. In turn, these distractions delay attack and provide thornbill nestlings with an opportunity to escape. This sophisticated defence strategy exploits the complex web of interactions among multiple species across several trophic levels, and in particular exploits a predator's ability to eavesdrop on and respond appropriately to heterospecific alarm calls. Our findings demonstrate that prey can fool predators by deceptively mimicking alarm calls of harmless species, suggesting that defensive mimicry could be more widespread because of indirect effects on predators within a web of eavesdropping.

Branislav Igic, Jessica McLachlan, Inkeri Lehtinen and Robert D. Magrath. 2015. Crying Wolf to A Predator: Deceptive Vocal Mimicry by A Bird Protecting Young. Proceedings of the Royal Society B. 282(1809). DOI: 10.1098/rspb.2015.0798.

Brown Thornbill Mimics Alarm Calls of Other Birds to Scare off Predators | Biology |