A CHRONIC shortage of female swift parrots has led to love triangles, sneaky sex on the side, increased fighting between males and fewer babies, according to scientists from ANU. 

Swift parrots, which are critically endangered, used to, roughly, have an equal ratio between male and female, but over time it has increased to almost three to one since the introduction of a tiny predator to Tasmania in the 1800s – the sugar glider.

Sugar gliders can access nest hollows and kill female swift parrots while they incubate their eggs. More than half of the females die each year at their breeding grounds in Tasmania.

Lead researcher Prof Rob Heinsohn said the research team studied the swift parrot mating system using molecular techniques and 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.

The study, which is published in the “Journal of Animal Ecology”, found mate sharing was not beneficial for anyone in the “ménage à trois”.

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

The researchers used population modelling to isolate the impact of lower reproduction due to mating in trios.

“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 five per cent,” he said.

“We were aware of many nests where an extra male would hang around and harass the female, but were absolutely flabbergasted to find that the females were engaging in sneaky sex with them.

“We think the females are having sex with the other males for a range of reasons, but probably the main one is just to get them off their backs.”

Prof Heinsohn said both sexes suffered whenever mate sharing occurred.

“The obvious costs to females are being harassed by too many males, while males are forced to fight for females to mate,” he said.

“The overall population takes a hit, as a consequence, because they are having fewer babies.”

Prof Heinsohn said the study was important because it teases apart how individuals in populations may be affected differently by introduced predators.

“Especially how the loss of so many females can change the balance of the sexes, as well as the whole mating and social system,” he said.

He said swift parrots were not the only species where the fabric of society was threatened by too few females.

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