Information theory reveals that individual birds do not alter song complexity when varying song length

The application of information theory to birdsong research provides insights into the internal organizational structure of the communication systems of nonhuman animals, which can be integrated with behavioural data. We applied information theory to test for trade-offs between two birdsong features, complexity and duration, which may be costly for the individual. We determined and quantified the internal structure of the song in a wild passerine, the spectacled warbler, Sylvia conspicillata, and tested for differences in song variability between song bouts of different lengths. We found (1) no correlation between song bout length and internal complexity, since song bouts of different durations presented the same degree of complexity, (2) a high potential for communication capacity because of the equilibrium between the unification and diversification of the song repertoire, and (3) a communication system that followed a first-order Markov chain with a high degree of variation and individuality, giving rise to a complex and highly variable song. The fact that both short and long songs showed high internal complexity suggests that no clear constraint exists between these features, and that the spectacled warbler may sing intrinsically complex songs throughout the display. This complexity is enhanced by the lack of sharing among individuals, which would otherwise homogenize songs and constrain individual innovation ability.