Seasonally variable and sex-specific motivation for cooperative resource defence in a duetting songbird—The Chubb's cisticola

Duets are one of the most complex forms of acoustic displays, involving two individuals vocalising in a coordinated way to defend resources cooperatively. However, motivation to defend resources may be sex specific and variable in time and space, which should be reflected in the changes in duet singing behaviour. We experimentally examined whether the motivation to defend resources is sex specific and seasonally variable in an Afrotropical, sedentary songbird living in pairs or small groups—the Chubb's cisticola. We broadcasted songs of males, females and duets in a random order, both at the beginning and the end of the breeding season. We quantified the birds' responses from the perspective of the entire group as well as its individual members. We found cooperative responses to each type of intruder, with the birds flying and singing. Groups consisting of two to four individuals responded significantly more strongly towards intrusions by females than pairs, and at the beginning than at the end of breeding season. Solo syllables comprised less than 4% of all recorded syllables, and in most cases were sung within a song also containing duet and chorus syllables (only 12 from 1773 songs analysed were solo songs). Males sang solo syllables more often than females (83% vs. 17% of all solo syllables) and did it significantly more often towards intruding females than males or pairs. The number of solo syllables sung by females was independent of treatment and season. Together, our results suggests that Chubb's cisticola uses duets to cooperatively defend resources located within its territory, but the intensity of reaction is sex specific and seasonally variable. We suggest that defended resources could be mates, group stability or reproductive effort. Looking at the function of duets from the perspective of the group and its particular members, various stages of the season and different resources defended by birds allow for a better understanding of cooperative singing.