Frequent duets, rare choruses, and extremely rare solos: year-round singing behavior in Chubb's Cisticola

Our current understanding of the function of coordinated acoustic displays usually comes from studies conducted over a short period of the breeding season. However, the function of particular types of vocalizations may vary according to sex and context, and such displays can extend beyond the time of reproduction. To fully understand this phenomenon, analyses of year-round singing behavior are required. In the current study, we focused on a small, year-round territorial Afrotropical songbird, Chubb's Cisticola (Cisticola chubbi). We analyzed the structure of songs during the breeding season as well as year-round changes in the proportion of solos, duets, and choruses to investigate the potential function(s) of each type of vocalization. We found that: (1) females produced whistling notes, while males generated trilling ones; (2) up to five individuals formed coordinated choruses, and (3) individuals were always near to each other during cooperative singing. Over the course of a year, the majority of syllables recorded were duets (82%), with rarer choruses (16%) and extremely rare solos (2%). Outside of the breeding season, males produced the most solos, while females produced more at the beginning of the breeding season. The proportion of choruses was highest at the end of breeding season. Frequent year-round production of duets and choruses strongly supports territory defense as the main function of joint singing, while the highest proportion of choruses at the end of the breeding season suggests that offspring take part in the chorus. To better understand cooperative singing, it is essential to extend our looking beyond the breeding season.