The dawn chorus of the great tit can be interpreted from a functional point of view in terms of the following factors. (i) Climatic and other physical conditions in the early morning are unfavourable for foraging and favourable for acoustic communication. (ii) Overnight accumulation of territories favours early morning invasion by potential settlers. (iii) The combination of (i) and (ii) favours early morning territorial defence, including song. A laboratory experiment designed to investigate the proximate causes of allocation of time by territorial great tits to foraging vs. territorial activities (including song) showed that birds are more responsive to intruders when food availability is low. We discuss the relevance of our results to the dawn chorus in other animals and in other geographical regions.