Solo songs, duets and territory defence across seasons in female Galápagos yellow warblers, Setophaga petechia aureola

Although the function of birdsong 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 the neglect of female song. We examined the female song in Galápagos yellow warblers, Setophaga petechia aureola , and show for the first time that it is widespread in the form of solo songs as well as duets. We interrogated the seasonal patterns and functions of female song by carrying out simulated territory intrusions using playbacks of male, female or duet songs during breeding and nonbreeding 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 nonbreeding season and predominantly in male-led duets. Although females were strongly aggressive towards female song playback, they gave the weakest singing response to it. 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 signalling aggression in territory defence or intrasexual competition but may have other functions.