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In many avian species, both members of a pair or all members of a breeding group sing, often in duets or choruses. Previous studies indicate that duets often function in territorial defence. However, other potential functions, such as acoustic mate-guarding, need further study, because most species that have been studied have low or unknown rates of extrapair paternity (EPP) and theoretical models predict species with moderate rates of EPP will guard more than those with very low or high rates. We conducted behavioural observations to describe the form of duets and test hypotheses for the function of duets in breeding groups of Red-backed Fairy-wrens (Malurus melanocephalus), a species with moderately high rates of EPP. Within duets, notes overlapped in frequency and time, rather than being antiphonally coordinated. Duets and choruses occurred more in pre-breeding than other stages of breeding, and more in inter-group than intra-group contexts, supporting a territorial defence function. Contrary to predictions of an acoustic mate-guarding function, males did not duet with mates more when they were fertile. Likewise, functions of breeding coordination and group establishment were not supported. Thus, Red-backed Fairy-wrens may use duets for territorial defence rather than mate-guarding, despite their moderately high rates of EPP. Investigating how vocal behaviour varies across different mating systems allows us to test theoretical predictions and determine how mating system affects the evolution of signalling strategies in birds and other taxa.

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