Personality predicts song complexity in superb fairy-wrens

In birds, singing behaviours play a critical role in mating and territory defence. Although birdsong can signal individual quality and personality, very few studies have explored the relationship between individual personality and song characteristics, and none has investigated this in females. Here, we examined the relationships between song complexity and two ecologically relevant personality traits (exploration and aggressiveness) in wild superb fairy-wrens (<i>Malurus cyaneus</i>), a species in which both sexes learn to produce complex songs. First, we assessed personality in males and females (including juveniles) by quantifying their exploration behaviour (novel environment test) and aggressiveness (mirror stimulation test) during short-term captivity. After birds were released, we recorded their songs over several months to assess individual variation in song complexity (i.e. element types per song and syllables per song) in relation to personality. Regardless of their sex or life stage, individuals that were more exploratory had more element types per song. Additionally, more aggressive individuals produced songs with fewer syllables, and more aggressive fledglings, but not adults, had more element types per song. Our study supports the idea that both male and female birds can advertise their personality when singing, which may be important for mate choice.