The role of vocal consistency in bird communication, a case study in the blue tit (Cyanistes Caeruleus)

Sierro, Javier and Hartley, Ian and de Kort, Selvino (2022) The role of vocal consistency in bird communication, a case study in the blue tit (Cyanistes Caeruleus). PhD thesis, Lancaster Environment Centre.

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Abstract

Birdsong is a multidimensional acoustic signal used for communication during intra- and intersexual interactions. The anatomical and behavioural adaptations related to sophisticated phonation skills indicate there are directional selection pressures shaping phonation mechanisms in songbirds. The level of neuro-motor challenge displayed while singing is normally referred to as vocal performance of song. Theoretical and empirical work has shown that vocal performance is related to individual quality and is meaningful during communication. Here, I use the blue tit (Cyanistes caeruleus) as a model species to investigate the communicative value of vocal performance in birdsong with particular emphasis on vocal consistency. Vocal consistency, as the ability to reproduce the same song element with minimal variation, has been proposed as an honest signal of individual quality in songbirds. In line with this hypothesis, I found that higher vocal consistency in males was preferred by females and was associated with increased reproductive success. I also provide evidence to validate the Spectral Cross-Correlation method used to measure vocal consistency, by showing it is sensitive to acoustic variation in natural birdsong. Although vocal consistency may be meaningful by itself, I built a multi-species database to investigate possible interactions of vocal consistency with other aspects of song. The results suggest there is a general trade-off between vocal performance within song and song length in birds. Following this finding, I conducted two playback experiments that showed that 1) long songs elicited a stronger response from territorial males than short songs but 2) that long songs with reduced vocal consistency triggered a lower response from territorial males, compared with short songs of high vocal consistency. These playback experiments indicated there is communicative value in the performance trade-off between song length and vocal consistency. Finally, I provide evidence that female song is frequent in blue tits and contextual behaviour data suggest it plays a role in intrasexual competition, similar to male song. However, during dawn chorus, males but not females were observed singing. These contextual differences indicate that female song may not be subjected to strong selection pressures related to seeking copulations. I hypothesize that such functional differences may relate to sexual differences in song parameters as females sang with lower song diversity and lower vocal performance than male.