Mechanisms of song production in the Australian magpie
Australian magpies (Gymnorhina tibicen) are notable for their vocal prowess. We investigated the syringeal and respiratory dynamics of vocalization by two 6-month-old males, whose songs had a number of adult features. There was no strong lateral syringeal dominance and unilateral phonation was most often achieved by closing the syringeal valve on the contralateral side of the syrinx. Unlike other songbirds studied, magpies sometimes used an alternative syringeal motor pattern during unilateral phonation in which both sides of the syrinx are partially adducted and open to airflow. Also, in contrast to most other songbirds, the higher fundamental frequency during two-voice syllables was usually generated on the left side of the syrinx. Amplitude modulation, a prominent feature of magpie song, was produced by linear or nonlinear interactions between different frequencies which may originate either on opposite sides of the syrinx or on the same side. Pulse tones, similar to vocal fry in human speech, were present in some calls. Unlike small songbirds, the fundamental of the modal frequency can be as low as that of the pulse tone, suggesting that large birds may have evolved pulse tones to increase acoustic diversity, rather than decrease the fundamental frequency.
Despite the importance of Oscine songbirds as an experimental model in which to study vocal learning and vocal communication, the syringeal and respiratory mechanisms controlling song production have been studied in only a few of the approximately 4,000 species in this diverse suborder. Even from this small sample it is apparent that different taxa have evolved different vocal motor strategies with which to produce their species-typical songs. Each of the several taxa for which there are data, uses the two sound sources in its bipartite syrinx in different ways to achieve the salient acoustic effects of its species' song (Suthers 1997, 1999; Suthers and Zollinger 2008). When a vocal mimic, the northern mockingbird (Mimus polyglottos), mimics heterospecific song, the bird uses syringeal motor patterns that are similar to those of the tutor species (Zollinger and Suthers 2004). It thus appears that each species has evolved the optimal vocal motor pattern for producing its particular song. Comparative studies of song production can provide insights into the nature of performance constraints on birdsong and their influence on the evolution of song diversity.
