Australasian Science: Australia's authority on science since 1938

Parrot Infanticide Favours the Female Chicks

By Stephen Luntz

Eclectus parrots are one of only three species known to engage in sex-selective infanticide.

Aspects of the remarkable sex-selection behaviour of maternal Eclectus parrots have been explained in Current Biology, but tantalising mysteries remain.

The parrot, which is native to Cape York and Papua New Guinea, will sometimes kill its offspring. “It’s interesting in itself because infanticide is weird – why do you have babies and then kill them?” asks lead author Prof Robert Heinsohn of the Australian National University.

While it is not unknown for animals to kill their young, Eclectus parrots are, with humans and antechinuses, the only species known to engage in sex-selective infanticide.

It took Prof Heinsohn and his team 10 years to produce a sufficient data set to be confident that they understood the birds’ logic.

Female parrots always lay two eggs in a season and are solely responsible for guarding the nest and caring for the young. However, they rely on males to provide them with food.

Heinsohn observed that infanticide only occurs when one baby is male and one female. In nearly 20% of such cases the mother will kill the male. Whether she does depends on the quality of her nest.

The parrot nests in hollows, but Heinsohn says: “Some are really good for nesting in, some are poor. The poor ones have a habit of flooding in heavy rain, drowning the chicks or eggs inside.”

A mother with a bad nest will sometimes kill a male baby so that she can lavish attention on the female, accelerating its development so it is ready to leave the nest before a flood.

The down of infant male and female Eclectus parrots differs in colour. Heinsohn doubts that the same behaviour occurs in many other species, as most would not be able to tell the sex of their offspring until much time and energy has been invested.

Female chicks fledge up to a week before their brothers, so the practice makes some sense. Nevertheless, the result is a surfeit of female parrots, which is disadvantageous for the species.

Nesting hollows, even poor ones, are in such short supply that many females do not get to mate for lack of a place to lay their eggs. “They’ll fight, sometimes to the death, for a hollow,” Heinsohn explains.

“This unusual behaviour affects the balance between the sexes in the adult Eclectus population, and should make the over-produced female chicks less valuable,” Heinsohn says. “However, provided you don’t do it too often, the benefits of producing at least one surviving chick are such that you can get away with it.”

Males are also more common than nesting hollows, but the parrots deal with this by having a harem of up to four males attending to each female. “She’ll mate with all of them, and the eggs may be from different fathers,” Heinsohn says. “Over a number of years different males may get to be fathers.”

Lacking paternity tests, all of the male parrots that have mated with a female will supply her with food while she is guarding the young.

Heinsohn was originally drawn to study the parrot by learning of observations that parrots in captivity would produce long strings of offspring of one sex or the other – sometimes 30 males in succession. Clearly the parrot has a similar capacity to Gouldian finches to partially control the offspring of its young (AS, June 2009, p.11).

In the wild, parrots with bad nest hollows will produce slightly fewer males at hatching, but Heinsohn says the infanticide represents “fine tuning” for inadequate control.

Heinsohn says it is still not apparent what cues are being given to birds in captivity that lead to much stronger sex selection at conception.