The social system of Sharpe’s Wren (Cinnycerthia olivascens): fluid group composition in a cooperative breeder

El sistema social del cucarachero de Sharpe: fluidez en la composición de grupos

Abstract

Cooperatively breeding birds live in groups that normally originate in the retention of offspring, which delay dispersal and stay in the parental territory, helping to raise close kin. Group transfers usually occur when individuals disperse to obtain re-productive positions. Between 1995 and 2000 we studied the social system of Sharpe's Wren (Cinnycerthia olivascens) in an Andean forest. Wrens lived all the time in groups of up to seven individuals that maintained year-round, all-purpose territories (mean = 7.6 ha) and raised offspring cooperatively. Each group had only one breeding pair and produced 1-3 fledglings. Group composition was highly variable. Birds of all age classes frequently joined and abandoned groups. Birds became helpers when joining new groups. In one case a female breeding vacancy was filled by a bird that had joined the group two and a half years earlier. Four focal groups each had up to 20 different, temporary members in three years of observation, and only the breeding pair was permanent. Seven of eight breeding females and one male (of 35 banded adults) had frontal patches and periocular rings of white feathers, suggesting that this characteristic is related to breeding status (but not necessarily age). Groups maintained three or more roosting nests simultaneously and built new nests continuously. Flea infestations may have been a cause of nest abandonment. Fluidity in group membership suggests that benefits of sociality in this species are not related to indirect fitness, nor to direct benefits that hinge on life-long associations between individuals. Instead, group-living may be related to the need to constantly build new, massive nest structures.