The study of female song in a socially monogamous territorial bird provides an opportunity to understand selective pressures specific to females. We looked at the influence of female song behaviour on the quality of the mate and breeding territory acquired. We examined how female song and singing behaviour correlated with body condition and reproductive success. Finally, we determined whether there is a trade-off between female song production and investing in offspring. We investigated female song in the New Zealand endemic bellbird (Anthornis melanura), a passerine breeding in temperate forest habitats. We monitored bellbird breeding activity on Tiritiri Matangi Island between 2007-2009 and 2012-2014. The density of bellbirds within the study area was approximately 4.7 pairs/ha. The frequency of female singing bouts increased through the breeding season, whereas male singing bouts decreased. Female song production was highest during egg-laying and late chick-rearing and lowest during incubation and early chick-rearing. Female bellbirds were more active than males in parental care and a female’s song complexity and singing rates were positively associated with the amount of male parental care but not male body condition. Females in better condition sung more frequently and used syllables with lower frequencies.  The more aggressive female responses during courtship and chick-rearing reflect the costs of territory intrusions and we suggest a trade-off between paternal care and territorial defence. Female defence, particularly towards female neighbours, may provide a mechanism for preventing polygyny and reducing the risk of loss of male parental care. It is unclear why male body size/condition was not associated with reproductive success and we speculate that extra-pair paternity must be considered to understand male reproductive success for this species. We conclude that a bellbird female’s singing behaviour is fundamental to her reproductive success and her defence of a quality territory and mate.