Vocal mimicry in the spotted bowerbird Ptilonorhynchus maculatus

Abstract

Vocal mimicry is well documented in songbirds, yet the function of this behaviour is poorly understood. I studied vocal mimicry in a wild population of male spotted bowerbirds Ptilonorhynchus maculatus to determine whether there was any support for the proposed functional hypotheses invoked to explain this behaviour.

I collected observational data to determine what species male bowerbirds mimicked and how their mimetic repertoires related to the acoustic environment. Spotted bowerbirds preferentially mimicked the vocalisations of aggressive species, which is consistent with mimicry acting to deter predators or competitors (Batesian mimicry). However, these sounds were also relatively simple in terms of their structure, and may be mimicked purely due to their simplicity and similarity to the species-specific hiss. A survey of mimetic repertoires at three geographically isolated populations revealed a similar pattern in model choice: mimetic repertoires were predominantly composed of aggressive and predatory species but these sounds were also structurally simple.

To test whether mimicry was used in a Batesian context I determined what contexts mimicry was produced in. Consistent with predictions, I found that males did not increase their mimetic rate in the presence of conspecifics but did increase their mimetic rate in response to human activity around the bower.

To determine how mimetic sounds are acquired in this species, I compared the mimetic repertoires of individuals within a population and found that males with bowers closer together mimicked more of the same species than did males with bowers that were further apart. Closer inspection of two of these mimicked sounds revealed that neighbouring males did not produce structurally similar mimicry,which suggests that mimetic sounds are learned directly from the species being mimicked.

Males did not increase their rate of species-specific vocalisation when mimetic rate increased, so these vocalisations are unlikely to serve the same function.

Males increased their rate of species-specific hissing when in the presence of conspecifics and this vocalisation is likely to function in intraspecific communication. Males also produced ‘advertisement’ calls when alone at the bower that are likely to attract females to the bower or deter rival males. These vocalisations are a long distance signal that varied in structure in three populations of bowerbird. I discuss potential explanations for geographic variation in the structure of bowerbird vocalisations. Vocalisations may be part of the multi-component sexual signal produced by bowerbirds, but I found no relationship between any aspect of male vocalisation and predicted mating success, so these vocalisations are unlikely to indicate male quality to potential mates or rival males. In conclusion, it seems most likely that mimicry in this species is used to deter predators or competitors, but I cannot exclude the hypothesis that mimetic sounds are learned as a result of their relative simplicity and salience in the acoustic environment. Furthermore, I have shown that mimetic sounds in this species are most likely acquired directly from the species being mimicked. These findings are a useful step towards understanding the function and evolution of this fascinating behaviour.