Male parrots from an endangered monogamous species are running out of mates, so now they're hitting up breeding females for sex

A small possum is ruining the sex life of a critically endangered Australian parrot species, putting them in even greater danger of extinction, as the previously monogamous birds find themselves in the avian version of a threesome.

The sugar glider is to blame. Cute, but deadly, the predators were introduced to the swift parrot breeding grounds in Tasmania in the 1800s, where they get into the nest hollows and kill female parrots while they're incubating their eggs. Around half of the females swift parrots die annually as a result.

As result, the previously balanced ratio of male to female swift parrots has plummeted to three blokes for every one female.

Scientists at The Australian National University (ANU) found a chronic shortage of females led to love triangles, sneaky sex, increased fighting between males and fewer babies as part of a six-year study of the birds around Hobart, Tasmania.

Lead researcher Professor Rob Heinsohn, from ANU's Fenner School of Environment and Society, said the research team found more than half of the nests had babies with more than one father.

“This is remarkable for parrots because most species are monogamous,” he said.

And it's not helping with the population's recovery.

“The overall number of babies born fell whenever the sex ratio became more male-dominated and shared paternity went up,” Professor Heinsohn said.

“Although most population decline was directly attributable to sugar gliders killing nesting females, the impact of conflict and lower success from shared mating reduced the population by a further 5%.”

Professor Heinsohn said that while the research team were aware of many nests where an extra male would hang around and harass the female, they were "absolutely flabbergasted to find that the females were engaging in sneaky sex".

And while they concluded there were a range of reasons for it happening, the main one will be familiar to humans, especially the #metoo movement.

Basically, the boys are pestering the girls for sex until they give up and let them "just to get them off their backs” Prof. Heinsohn said.

And between the blokes scrapping and female exhaustion from harassment, the consequence is fewer chicks.

There are several lessons playing out for a number of species, including humans, from this study, which is published in the Journal of Animal Ecology. The first is how a change in the balance of the sexes can influence mating and social systems.

Prof Heinsohn said swift parrots aren't the only species threatened by too few females.

“It’s happening in other birds, reptiles, and even humans in some parts of the world,” he said.

Scientists only discovered how big a problem sugar gliders were in 2014 - concluding the swift parrot could be extinct by 2031 as a result - and the ANU team has been working on Bruny Island, south of Hobart, where the marsupial isn't found, to improve the environment and the chance of the critically endangered parrot surviving down there.

An estimated 2000 mature birds survive in the wild.