In my dissertation research, I investigated how birds sing, and what physical and physiological limitations may constrain and shape their vocal behaviour.
The diverse vocal performances of songbirds are produced by coordinated patterns of activity in muscles controlling two separate sound generators on the right and left sides of their vocal organ, the syrinx. But how do songbirds use these two sound sources to produce their acoustically and temporally complex vocal communication signals? I used northern mockingbirds (Mimus polyglottos) as a model species to investigate peripheral sources of vocal complexity, and to test hypotheses about putative constraints on vocal production.

In the experiments detailed in my thesis, I show that when a vocal mimic accurately copies the song of another species it must also use the same vocal motor pattern employed by the model species. Vocal motor “mistakes” or deviations from the stereotyped motor patterns associated with the most acoustically similar mimicries of each sound type, resulted in predictable deviations from accurate copies of tutor sounds.

I also present new physiological evidence suggesting a mechanical and acoustic coupling between the two sides of the syrinx. Mockingbird mimicries of synthesized tutor sounds designed to test the ability of the two sides of the syrinx to simultaneously generate different sounds, reveal that although the two sides of the syrinx are theoretically independent, there are significant constraints on how different the contribution from the two sides can be.

Finally, I found that nonlinear dynamics within the syrinx can give rise to phenomena such as subharmonics, frequency jumps, biphonation and chaos that further increase the acoustic complexity of mockingbird song. By identifying the features of song most difficult for mockingbirds to produce, we can gain insights into the physical and physiological limitations on song production and further the understanding of the types of selective forces that may drive the evolution of vocal communication signals.

If you're still interested and awake and want to read more, you can download my dissertation by clicking this link:
Performance Constraints and Vocal Complexity In Birdsong