We studied seasonal and diurnal variation of singing activity in a single-song repertoire species, the savannah sparrow (Passerculus sandwichensis), to explore the possible functions of the song. We observed a total of 47 territorial males (11 over three years, 28 over two years) on a daily basis. For all paired and unpaired males, and across all stages of the breeding season, we recorded total number of songs produced (singing persistence), the number of instances of aggression per 10-minute period in each focal male, and the number of songs per 3-minute period (singing intensity). Male savannah sparrows showed different singing activity in different behavioural contexts: 1) unpaired males sang more persistently, but less intensely than paired males; 2) paired males markedly reduced their singing persistence, but showed higher singing intensity, especially in the evening; 3) singing intensity of paired males peaked during the egg-laying period. The different patterns of singing activity in relation to time of day, nuptial status, and female breeding stage suggest that though the birds each possess only a single song type, the differential activities may play important roles in intra- and inter-sexual communication systems: it is argued that 1) intense singing by paired males in the evening plays a role in territory defense, while persistent singing by unpaired males in the morning plays a role in mate attraction; 2) especially high evening singing activity during the egg-laying period may relate to changed female behavior at the nest associated with the onset of incubation.