Vocal tract filtering and sound radiation in a songbird

Brian S Nelson,Roderick A Suthers,GabriëL J L Beckers

doi:10.1242/jeb.01378

Brian S Nelson, Roderick A Suthers + Show 1 more

Open Access

https://doi.org/10.1242/jeb.01378

Copy DOI

Abstract

Bird vocalizations resonate as they propagate through a relatively long trachea and radiate out from the oral cavity. Several studies have described the dynamics with which birds actively vary beak gape while singing and it has been hypothesized that birds vary beak gape as a mechanism for varying vocal tract resonances. Nevertheless, few studies have attempted to quantify the effects of beak gape on vocal tract resonances. We replaced eastern towhee, Pipilo erythrophthalmus L., syringes with a small speaker and obtained recordings of frequency sweeps while rotating each subject in a horizontal plane aligned with either the maxilla or mandible. We describe vocal tract resonances as well as how sound radiates as a function of beak gape. Results are inconsistent with the hypothesis that songbirds vary beak gape as a mechanism for 'tracking' fundamental frequencies in vocalizations. Instead, decreases in beak gape seem to attenuate resonances that occur between approximately 4 and 7.5 kHz. We propose that songbirds vary beak gape as a mechanism for excluding and/or concentrating energy within at least two distinct sound frequency channels.

Highlights

Songbirds produce vocalizations using syringeal structures that are located at the caudal end of a trachea that extends several centimeters into the thoracic cavity
Resonances within the vocal tract seem to further amplify sound frequencies near approximately 2 and 5.5·kHz, relative to amplitude levels measured from the sound source alone or when the sound source was inserted into a baffle, but not over intermediate sound frequencies (Fig. 4A)
The Spectral peaks shift in frequency between ~4 and 5.5·kHz as beak gape varies (Fig.·5A)

Summary

Introduction

Songbirds produce vocalizations using syringeal structures that are located at the caudal end of a trachea that extends several centimeters into the thoracic cavity. Hoese et al, 2000; Nowicki and Marler, 1988; Podos et al, 2004; Westneat et al, 1993) In contrast with these theoretical predictions, experimental results reported for the European blackbird (Turdus merula) suggested that changes in beak gape might alter the amplitudes of sound frequencies above ~4·kHz (Fig. 1; Larsen and Dabelsteen, 1990). These authors do not describe vocal tract resonances. Several subsequent studies have reported results in which it remains unclear whether birds vary beak gape as a mechanism for ‘tracking’ fundamental frequency or as a mechanism for controlling amplitude above ~3–4·kHz (Fig. 1B; e.g. Goller et al, 2004; Nowicki, 1987; Podos et al, 2004; Suthers and Goller, 1997; Westneat et al, 1993)

Methods

Results

Discussion

Conclusion