What Behaviours arise from complex system-wide interactions between the body, nervous system, and the surrounding environment. Birdsong is one of the best-studied examples of a complex learned behavioural trait. The ability to produce complex songs is critical to the fitness of an individual. The aim of this project is to quantify the morphological diversity of the syrinx across the family Estrildidae and then compare it to the acoustic output to answer questions about the rapid radiation of songbird vocal diversity in general. Why The results from this proposal takes into account the system (the syrinx) and its interactions with the surrounding environment (from habitat acoustics to social structures) to understand the output (the song). Consequently, this proposal will have a broad impact on the fields of vocal communication and the evolution of sound production and vocal diversity in birds, as well as in humans and other mammals. How I propose an ecophysiology approach of cutting-edge ex vivo preparations, sub-micro resolution tomography, bioacoustics and contemporary evolutionary analysis to model how vocal complexity evolved in songbirds. SSR This proposal will have a broad impact on the fields of vocal communication and the evolution of sound production and vocal diversity in birds.