As you climb above the oppressively humid rainforest of Cape York Peninsula, you feel the refreshing breeze in your face; birds and butterflies whiz past. You break above the canopy and realise that, suddenly, everything’s below you.
It’s a privileged perspective that not many people will experience, but one that ANU professor Rob Heinsohn has spent much of the last 10 years admiring, perched on a wooden platform up to 30 metres above the ground, observing the mysterious eclectus parrot (Eclectus roratus) for months on end.
“I was terribly scared of heights at first,” he says. “But after the first season it has been replaced by the joy of tree climbing.”
The eclectus parrot is an anomaly of the animal kingdom, possessing features that defy evolutionary expectations. The males and females look so different from each other that for a long time people thought they were separate species. But these parrots have a much more unusual secret – they can choose their offsprings’ gender.
“There was a famous parrot in Chester Zoo that produced 30 sons before switching to produce 20-something daughters. Nobody knew why, but it told us that these parrots have some way of controlling the sex of their offspring,” says Heinsohn.
With 70 metres of climbing rope under one arm and a bird handling kit, laptop and spectrometer under the other, Heinsohn and colleagues set off to find out more.
They discovered that the tree hollows that the parrots nest in are few and far between, making them a very precious resource for breeding females. In fact, there’s only about one useable hollow for every two square kilometres of rainforest.
“The females are so scared of losing their hollow that they literally never leave it for the whole breeding season of up to eight months,” says Heinsohn. “They get away with it by relying on the males to bring them food.”
Heinsohn says it was the realisation that the females always laid two eggs that gave rise to their most fascinating discovery.
“We always observed nests with two eggs and they were usually both viable,” he says. “But the next time we climbed up , there would only be one chick.
“Then we worked out that the missing chick was always a male. In an act of sex-specific infanticide, the females were tossing the males overboard.
“We saw beak-shaped peck marks on the backs of their head or neck when we found their little corpses.”
Interestingly, the perpetrators were females with sub-standard nest hollows. Because of nest flooding during the heavy storms that pass over, these mums are stressed-out and time-short, says Heinsohn.
“We worked out that because female chicks move out of the nest more quickly than males on average, it actually pays off for the mothers to get rid of the male offspring simply so they can speed up the nesting process and produce at least one chick.”
Humans are the only other animals known to murder offspring on the basis of their sex.
“We now know that some female parrots kill their offspring in the wild; but we still don’t know why they do it in captivity,” says Heinsohn.
Thus, the mystery of the eclectus parrot continues.