Seasonal and diurnal variation of vocal behaviour in duetting White-browed Sparrow Weavers

Singing in tropical and Southern hemisphere birds often occurs throughout the year in the form of duetting, which is thought to be associated with year-round territoriality. White-browed Sparrow Weavers (Plocepasser mahali) are cooperatively breeding songbirds of Eastern and Southern Africa, which live in groups of 2–10 individuals and defend all-purpose territories year-round. Their vocal behaviour is extraordinary as all group members sing duet and chorus song, but the dominant male of the group, additionally, sings a long solo song at dawn that comprises a distinct syllable repertoire. To gain further insight into the functions of the different types of song, the present study aimed at providing detailed quantitative data on diurnal and seasonal variation in singing activity. Therefore, we monitored the song behaviour of five groups of White-browed Sparrow Weavers at several time points during the course of the day and during the course of the year in relation to the groups’ breeding activities and environmental variables such as photoperiod, temperature and rainfall. Diurnally, solo song was heard exclusively during the first hour of vocalization in the morning before the onset of duetting. Seasonally, solo singing was restricted to the summer months. No tight link with breeding activities was found. Solo song duration was related to the density of neighbouring groups but not to group size. Similarly, duet/chorus rate showed large variation between groups and was related to the density of neighbouring groups. Diurnally, duet/chorus rate was significantly higher at the time point wake-up than at other times of the day. Seasonally, there was no difference. The increased singing rate at the time point wake-up, when all birds are present in the territory, suggests that these songs might convey information about territory ownership, group size as well as sex and status of the individuals.