Group display in the socially monogamous Northern Double-collared Sunbird (Cinnyris reichenowi).

Several sunbird species (Nectariniidae) exhibit group display, but the function of this behaviour still remains unclear. We test two hypotheses (reproductive tactic and joint territory defence) to explain the function of this behaviour using data gathered on Northern Double-collared Sunbirds (Cinnyris reichenowi) from the Cameroon Mountains. Sunbirds exhibited group display throughout the breeding cycle. Groups of birds were mainly comprised of neighbouring pairs aggregated at territory boundaries. The frequency of group displays varied significantly among individuals and over the breeding cycle, with the highest frequency during the pre-breeding period and the lowest during egg-laying (∼female's fertile period). Time spent on group display was significantly longer at the end of breeding. Off-territory forays were recorded for both males and females, males significantly increased the time spent on these forays during incubation. Extra-pair copulations were recorded only in five cases for females (4%) suggesting that most off-territory forays were motivated by feeding on nectar in neighbouring territories. Therefore, we suggest that the behaviour is used for joint territory defence rather than as an information centre for choosing mates for extra-pair copulations. Its function for choosing primary mates is also unlikely, because group displays were mainly observed in complete pairs.