Last updated December 7, 2018 at 2:46 pm
Predation is turning once-monogamous birds into promiscuous partners.
A chronic shortage of females is causing a social system breakdown in a critically endangered Australian parrot, forcing a monogamous species to engage in extra-pair matings, and driving down survival rates of nestlings.
Sugar gliders (Petaurus breviceps) are small gliding possums whose natural range is the forests of eastern Australia. The gliders were introduced to the southern island of Tasmania in the 1880s. Although named for their habit of eating sweet sap from trees, they are also highly efficient predators.
At night, the sugar glider enters the nesting hollow of the swift parrot (Lathamus discolor) and kills and eats the sitting female, as well as any eggs or young that may be present. Each breeding season more than half of the female swift parrots die.
Until now, the genetic implications of this sex-specific predation have been unknown. Scientists at the Australian National University have used molecular techniques to study the parrots’ mating system, and the findings reveal that despite the parrots remaining in socially monogamous pairs, 50.5% of nests had young with shared paternity.
“This is remarkable for parrots because most species are monogamous,” says lead researcher Rob Heinsohn.
Extra pair mating occurs in other bird species, such as blue tits (Cyanistes caeruleus) and superb fairy-wrens (Maluris cyaneus). These infidelities are integral to the species’ mating system and actually confer advantages such as increased fitness through genetic variability.
For swift parrots, the opposite is true. The six year study examined 371 nestlings from 85 nests across the species’ breeding grounds, and found that “the overall number of babies born fell whenever the sex ratio became more male-dominated and shared paternity went up.”
Although the extent of shared paternity in nestlings prior to the introduction of sugar gliders is unknown, it is likely that chronic shortage of females is promoting the extra pair matings.
The ratio of males to females is estimated at approximately 70% male, and as a result, both sexes are suffering.
Males are fighting for females, and females are being harassed at the nest by solo males. Heinsohn says, “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.”
This sex ratio bias in swift parrots caused by an introduced predator provides researchers with an unusual opportunity to isolate the costs of shared paternity (or genetic polyandry of nestlings) on individual fitness and population viability of a critically endangered species.
The researchers used population modelling to assess the impact of conflict and lower breeding success from shared mating, and predicted these factors would reduce the already precipitously declining species by a further 5%.
Heinsohn said the findings were important because they reveal 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 concludes.
The research was published in the Journal of Animal Ecology.