Abstract
BackgroundBirdsong and human vocal communication are both complex behaviours which show striking similarities mainly thought to be present in the area of development and learning. Recent studies, however, suggest that there are also parallels in vocal production mechanisms. While it has been long thought that vocal tract filtering, as it occurs in human speech, only plays a minor role in birdsong there is an increasing number of studies indicating the presence of sound filtering mechanisms in bird vocalizations as well.Methodology/Principal FindingsCorrelating high-speed X-ray cinematographic imaging of singing zebra finches (Taeniopygia guttata) to song structures we identified beak gape and the expansion of the oropharyngeal-esophageal cavity (OEC) as potential articulators. We subsequently manipulated both structures in an experiment in which we played sound through the vocal tract of dead birds. Comparing acoustic input with acoustic output showed that OEC expansion causes an energy shift towards lower frequencies and an amplitude increase whereas a wide beak gape emphasizes frequencies around 5 kilohertz and above.ConclusionThese findings confirm that birds can modulate their song by using vocal tract filtering and demonstrate how OEC and beak gape contribute to this modulation.
Highlights
Birdsong is a complex vocal behaviour often considered to show striking developmental and structural similarities with human speech [1]
These findings confirm that birds can modulate their song by using vocal tract filtering and demonstrate how oropharyngeal-esophageal cavity (OEC) and beak gape contribute to this modulation
This filtering process takes place in the upper vocal tract by altering the dimensions of various resonance cavities within the vocal tract, like pharyngeal, oral and nasal cavity. This is achieved by moving articulators such as tongue, lips and lower jaw [3]. In contrast to this source-filter theory of human speech [4] it has been long thought that frequency and amplitude modulations of bird vocalizations are mainly produced by the avian sound source, the syrinx, and that vocal tract filtering as in human speech production plays a minor role in generating vocal complexity in birdsong [5]
Summary
Birdsong is a complex vocal behaviour often considered to show striking developmental and structural similarities with human speech [1]. These observations lead to the hypothesis that the bird’s vocal tract can act as an acoustic filter and be actively modulated [6] Motivated by these findings subsequent studies on potential vocal tract articulators showed that beak movements and gape width are correlated with frequency patterns in white-throated sparrows (Zonotrichia albicollis), swamp sparrows (Melospiza georgiana) [8], Darwin’s finches [9] and zebra finches (Taeniopygia guttata) [10]. All of the mentioned studies suggest that possible articulators such as beak and the expandable esophagus are likely to modulate birdsong, these data are predominantly correlational As such, they are insufficient to precisely assess the role of different articulators in vocal production since their effects usually cannot be separated from each other or from other factors such as variation at the sound source. We manipulated beak gape and OEC expansion and compared acoustic input with acoustic output in order to evaluate the significance of these possible articulators
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