Many birds who appear to mate for life are actually promiscuous and engage in what’s called extra-pair copulations. To avoid getting duped into raising another bird’s chicks, males might turn to aggression and vocal displays. And according to a new Biology Letters study, red-backed fairy-wren couples who sing together stay together. 

If females are mating outside their pair bond, males from species showing paternal care should employ strategies to minimize extra-pair pair paternity. These behaviors include physical aggression towards intruders to deter would-be rivals and duetting – the synchronous combo of male and female vocalizations. The latter might help the males by signaling commitment between pair members or perhaps by jamming the mating signal of females.

To test whether male paternity is linked to aggression or duetting, a Cornell team led by Daniel Baldassarre simulated territorial intrusions in a population of red-backed fairy-wrens (Malurus melanocephalus). This Australian songbird species has high rates of extra-pair paternity; males go out into neighboring territories to pursue extra-pair females. The team used artificial mounts and song playback to simulate intrusions in Samsonvale, Queensland, and then they recorded the aggressive and duetting responses of the territory holders. 

The team was able to assign genetic paternity to 181 out of 186 (or 97 percent) of the offspring: 47 percent were extra-pair young, and 60 percent of nests contained at least one extra-pair young. Males with faster, stronger duet responses were more likely to have an offspring of their own in the nest – which means they were cuckolded less often than males with slow and weak responses. Male physical aggression, on the other hand, wasn’t particularly helpful in preventing rivals from copulating and siring young with their mates.

There are several, non-mutually exclusive reasons why strong duet responses are associated with high paternity assurance. Males might be guarding their mates acoustically by masking female signals that would otherwise attract competitors. It’s also possible that duetting strengthens pair bonds if the female joins in to signal her commitment. And finally, duetting may simply serve as a “keep out” sign to prevent potential usurpers. While vocal communication can help a male assure paternity amongst promiscuous birds, aggression may be more effective at maintaining territory boundaries and defending resources (rather than mates).