This thesis investigates the singing behaviour of coal tits (Parus ater) from two different study sites. A background to the functional significance of songs in a broad context is given in Chapter 1. This chapter also briefly introduces the biology of coal tits. Chapter 2 describes more general aspects of coal tit songs, and evaluates and compares the variability in song structure of two coal tit populations. The study has looked at the distribution of songs within populations, and at the effect of distance between individuals on the composition of their repertoires. The results are presented in Chapter 3. A new hypothesis has been proposed to explain the significance of song repertoires, namely the anti-exhaustion hypothesis. The hypothesis was tested on coal tits, and the results are presented in Chapter 4. Using playback experiments the singing interaction in coal tits was investigated. Two experiments were carried out in order to test the predictions that a bout of song could give information about individual's fitness, and that song length might indicate individual 'strength'. The results are presented in Chapter 5.