Context of female bias in song repertoire size, singing effort, and singing independence in a cooperatively breeding songbird

Among animals with conventional parental care roles, female bias in expression of elaborate sexually or socially selected traits is rare. However, such cases are instrumental for understanding selection pressures on females. Here, I present observational data showing that among mated pairs of cooperatively breeding stripe-headed sparrows, Peucaea r. ruficauda, singing behavior is broadly female-biased. Unlike in all other songbirds described to date, paired females outpaced males in singing effort, singing independence, and complexity of displayed repertoire. Social context of increased singing effort and song mode specialization suggested a strong influence of resource defense on females relative to males but did not exclude mate defense as an additional function. Females had larger song repertoires for territorial behavior, and males possessed a repertoire for mate acquisition that females did not. Males only equaled females in singing effort while singing in the mate acquisition mode. The female song bias toward aggression supports a previous study which concluded that resource defense is a strong selective pressure on female song evolution, given that female song response to simulated territorial intrusions was stronger than male response. Because stripe-headed sparrows remain pair-bonded and territorial year-round for multiple years, breeding females may be the dominant singers for a significant portion of their lives. Cases of female song bias are rarely noted in the literature, but increasing study of monomorphic, tropical, and Austral species may increase their number and change our perception of how elaborate, sex-specific traits evolve.