To test a new method of quantitatively analysing complex animal communication systems, we applied information theory concepts to the notes repertoire combined with temporal parameters of the Rufous-bellied Thrush Turdus rufiventris song. Like most Turdus thrushes, Rufous-bellied Thrushes are remarkable for their long, varied and melodious songs. For the analysis of the species repertoire, we used recordings of 44 individuals from 24 localities covering its full geographical range. We measured the repertoire size, note duration and rhythm (frequency of note utterance), and combined these parameters with the Shannon entropy values calculated for each individual. Although individuals maintain species-specific recognition capacity, we found a large variation between their song parameters and show that information theory can be useful to analyse large and varied animal vocal repertoires. In order to reduce the heterogeneity of our sample and elucidate the breath of the observed variation, we used mathematical tools based on information theory to create an index integrating the values of entropy and rhythmicity of the individual songs. To define the structure and organisation of this species song, values of the conditioned entropy were analysed to establish individual sequences and results were synthesised using cluster analysis. Although its song characteristics challenge human understanding of animal communication, the Rufous-bellied Thrush recognises itself as a species and successfully establishes intra-specific communication and all necessary biological functions. Our application of information theory to the structure and organisation of Turdus rufiventris song allows the identification of the chaotic behaviour of this communication signal, opening the way for further analysis which could demonstrate more clearly the function and codification of complex sound communication systems.