Solo songs, duets, and territory defence across seasons in female Galápagos Yellow Warblers (Setophaga petechia aureola)

While the function of bird song has been well studied in male songbirds, the function of female song is less well understood. This is partly due to a historical view of females occupying a passive role compared with males, which led to ignoring female song even in some well-studied species. We report one such case in yellow warblers (Setophaga petechia), as no other study investigated female song in 35 years after the first documentation of solo female songs. We interrogate the seasonal patterns and functions of female song in Galápagos yellow warblers (Setophaga petechia aureola), in which females perform solo song and produce duets with their male social partners. We carried out simulated territory intrusions by broadcasting male, female, or duet songs during breeding and non-breeding seasons, and conducted a territory retention survey for over a year. We measured the association between aggressive response and singing behaviour, sex-specific patterns of response, and territory retention across years. Females sang mostly during the non-breeding season and predominantly in male-led duets. Although females were strongly aggressive towards female song playback, they gave the weakest singing response towards them. There was no association between female aggressive behaviour and song output in response to a simulated intruder. Moreover, the probability of territory retention across years was not explained by song output or aggression in response to intruders, though evidence for this was weak due to the small sample size. We suggest that female song in this year-round resident island system does not function for territory defense or intrasexual competition, but may have other functions.