Adulterous male stitchbirds who sneakily father chicks with coupled-up females could prove key to saving the endangered little natives.

There are thought to be only 2000 hihi left in our forests, and a single disaster could wipe them out completely.

Now, a study led by the Zoological Society of London has pinpointed an unlikely saviour for the species - nest-hopping Casanovas.

Called "floaters", these bachelors don't hold any breeding territories or couple up with females but this doesn't stop them from coercing already attached females into mating.

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They tend to be young males who roam around looking for love.

While they have much lower breeding success rates compared with their cuckold counterparts, it is their ability to get females frisky that researchers say makes a real difference.

It's been shown to have a small yet significant impact on population size, as well as influencing sex ratio - factors that are important for maintaining genetic diversity and decreasing levels of inbreeding.

"Conservation management often discounts individuals that are thought to be unable to produce offspring, thinking that they have no effect on populations," said lead author Dr Patricia Brekke.

"We have shown that in hihi, floaters are able to reproduce and pass on their genes from one generation to the next, which helps with long-term survival of this endangered bird."

The researchers said more attention should be paid to the little lotharios, despite their being hard to study because they had "no fixed abode".

"Not taking floating individuals into account can undermine our conservation efforts."

Study co-author Dr Anna Santure, of the University of Auckland, said there had been similar recorded cases in several other bird species around the world, so it was "quite exciting" to demonstrate its occurrence in a threatened New Zealand population.

"Floating" has been observed in a range of species including fish, mammals and insects.

While a shameful act among us humans, in the animal kingdom it could be a successful breeding strategy as bachelor males avoided investing their energy into defending territory and looking after chicks, instead spending time searching for mates.

The rate of "beyond nest" liaisons was particularly high in tui, with 57 per cent of chicks sired by fathers other than the female's long-term partner, compared with a general average of 11 per cent among all monogamous birds. But even this was overshadowed by the hihi's extraordinary rate of around 70 per cent.