THE lovey-dovey monogamy which the untutored eye may perceive in pairs of songbirds, raising their young together in nests constructed by joint endeavour, has long been exposed by zoologists as a fantasy. Even by the adulterous standards of the ornithological world, though, the red-backed fairy-wren, an Australian bird, is a champion. According to a study just published in Biology Letters by Daniel Baldassarre of Cornell University and his colleagues, almost half of fairy-wren nestlings are not fathered by the male that is helping to feed them, and 60% of nests contain at least one such chick. For male fairy-wrens, then, trying to guarantee paternity is a continual struggle, and Dr Baldassarre and his team wondered which strategies worked best.

Broadly, a male fairy-wren has two options. He can try to discourage interlopers by beating them up, or he can woo his mate to encourage her not to stray. He does this by duetting with her. That both keeps her attention and may suggest to other males that she is, indeed, loyal.

To see which was the better approach Dr Baldassarre and his team ran some experiments in Queensland. They challenged male fairy-wrens with dummy interlopers who were given voice by recorded songs. They then compared the males’ typical responses to these artificial encounters with the paternities of actual nestlings subsequently raised, as estimated by comparing the youngsters’ DNA with that of their purported fathers.

In practice, all challenged males responded both by attacking the stuffed rival and by duetting with their mates. They did so, however, with varying levels of enthusiasm for each approach.

Altogether, in the course of several slightly different experiments, the researchers looked at the adults and offspring in 51 territories. Their results were intriguing. A male’s level of aggression towards the dummy made no perceptible difference to his chance, subsequently measured, of being a cuckold. His level of duetting, by contrast, made a big one. The best duetters had almost no offspring born of adultery inflicted on them.

Aggression, presumably, brings other advantages, otherwise it is hard to see why it happens. Probably, it serves to protect a territory (and thus its food resources) from predatory neighbours. In the matter of sexual fidelity, though, it seems to be useless. For female fairy-wrens, it is rather a mellifluous mate that is the key to domestic harmony and a loyal partnership.