Vocal tract articulation revisited: the case of the monk parakeet.

Birdsong and human speech share many features with respect to vocal learning and development. However, the vocal production mechanisms have long been considered to be distinct. The vocal organ of songbirds is more complex than the human larynx, leading to the hypothesis that vocal variation in birdsong originates mainly at the sound source, while in humans it is primarily due to vocal tract filtering. However, several recent studies have indicated the importance of vocal tract articulators such as the beak and oropharyngeal-esophageal cavity. In contrast to most other bird groups, parrots have a prominent tongue, raising the possibility that tongue movements may also be of significant importance in vocal production in parrots, but evidence is rare and observations often anecdotal. In the current study we used X-ray cinematographic imaging of naturally vocalizing monk parakeets (Myiopsitta monachus) to assess which articulators are possibly involved in vocal tract filtering in this species. We observed prominent tongue height changes, beak opening movements and tracheal length changes, which suggests that all of these components play an important role in modulating vocal tract resonance. Moreover, the observation of tracheal shortening as a vocal articulator in live birds has to our knowledge not been described before. We also found strong positive correlations between beak opening and amplitude as well as changes in tongue height and amplitude in several types of vocalization. Our results suggest considerable differences between parrot and songbird vocal production while at the same time the parrot's vocal articulation might more closely resemble human speech production in the sense that both make extensive use of the tongue as a vocal articulator.

Zebra finches and Dutch adults exhibit the same cue weighting bias in vowel perception.

Vocal tract resonances, called formants, are the most important parameters in human speech production and perception. They encode linguistic meaning and have been shown to be perceived by a wide range of species. Songbirds are also sensitive to different formant patterns in human speech. They can categorize words differing only in their vowels based on the formant patterns independent of speaker identity in a way comparable to humans. These results indicate that speech perception mechanisms are more similar between songbirds and humans than realized before. One of the major questions regarding formant perception concerns the weighting of different formants in the speech signal ("acoustic cue weighting") and whether this process is unique to humans. Using an operant Go/NoGo design, we trained zebra finches to discriminate syllables, whose vowels differed in their first three formants. When subsequently tested with novel vowels, similar in either their first formant or their second and third formants to the familiar vowels, similarity in the higher formants was weighted much more strongly than similarity in the lower formant. Thus, zebra finches indeed exhibit a cue weighting bias. Interestingly, we also found that Dutch speakers when tested with the same paradigm exhibit the same cue weighting bias. This, together with earlier findings, supports the hypothesis that human speech evolution might have exploited general properties of the vertebrate auditory system.

Zebra finches exhibit speaker-independent phonetic perception of human speech.

Humans readily distinguish spoken words that closely resemble each other in acoustic structure, irrespective of audible differences between individual voices or sex of the speakers. There is an ongoing debate about whether the ability to form phonetic categories that underlie such distinctions indicates the presence of uniquely evolved, speech-linked perceptual abilities, or is based on more general ones shared with other species. We demonstrate that zebra finches (Taeniopygia guttata) can discriminate and categorize monosyllabic words that differ in their vowel and transfer this categorization to the same words spoken by novel speakers independent of the sex of the voices. Our analysis indicates that the birds, like humans, use intrinsic and extrinsic speaker normalization to make the categorization. This finding shows that there is no need to invoke special mechanisms, evolved together with language, to explain this feature of speech perception.

Vocal tract articulation in zebra finches.

BACKGROUND: Birdsong 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 FINDINGS: Correlating 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. CONCLUSION: These findings confirm that birds can modulate their song by using vocal tract filtering and demonstrate how OEC and beak gape contribute to this modulation.