Do beak volume and bite force influence the song structure of sympatric species of seedeaters (Thraupidae: Sporophila)?

ABSTRACT

Song structure can be constrained by morphological characters such as beak size. Studies have shown that songbirds with larger beaks produce songs with a lower frequency, narrower frequency range and lower note rates than birds with smaller beaks. We tested whether beak volume and bite force constrain song parameters in three Neotropical seedeaters that show a gradient of beak size and force (Sporophila beltoni, S. caerulescens and S. hypoxantha). We also determined the relationship between body size and song structure. We evaluated the song structure of these species, measuring differences that may help them to segregate the acoustic space, since they occur in sympatry. These birds can be predicted to segregate the acoustic space to reduce overlapping of their songs, which is a possible result of the force of sexual selection. Individuals of each species had their beaks and bite forces measured and their own territorial songs recorded in the field. Only S. caerulescens showed a positive relationship between beak volume and song parameters (maximum frequency and frequency bandwidth). No song parameters showed an association with bite force or body size. The use of acoustical space seemed to be unique to each species with respect to note rate and song duration. Sporophila beltoni and S. hypoxantha showed the most differences from each other in all song parameters analysed. In contrast to other studies of finch species with conical beaks, our findings suggested that for these three species, beak volume and bite force do not limit the song structure.

KEYWORDS:

• Acoustic communication
• animal behaviour
• birdsong
• ecological segregation
• oscines

Acknowledgements

We are grateful to technician Samuel Corvello Vilar from the Physics Department of Pontifical Catholic University (PUCRS) for his help in surveying pressure-sensor adjustments and his expertise. The Brazilian National Research Council (CNPq) provided fellowships to N.S.P. and M.R. and a grant to C.S.F. (process 310022/2015-0). To Cornell Lab. (Macaulay Library) for lending the sound recording material to M.R.

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Funding

This work was supported by the Conselho Nacional de Desenvolvimento Científico e Tecnológico [310022/2015-0]