The mysterious behaviour of female Eclectus parrots killing their sons immediately after they hatch has been unravelled by a team of researchers from the Australian National University. Such sex-specific infanticide occurs in some human societies (where baby girls are killed) but is rare in other species.

In a paper published online in Current Biology this month, the team from the Fenner School of Environment and Society and the Research School of Biology describe the infanticide and the conditions surrounding the parrot’s choice to kill their young.



Male (green) Eclectus parrot feeding female (photo: M. Cermak)

“It’s interesting in itself because infanticide is weird - why do you have babies and then kill them?” remarked lead author, Professor Robert Heinsohn.

“Humans are the only other species that systematically kill their own offspring of one sex. But here’s a case in Eclectus parrots where we can show there is a very clear adaptive reason,” Professor Heinsohn continued.

Infanticide is easiest to understand when it involves killing the young of others, e.g. for food, or competitive reasons. But producing young is a costly business and it is very hard to understand why parents would kill their own offspring. In the case of Eclectus parrots, it's even harder to understand why parents systematically kill off one sex (males). Evolutionary theory tells us that it should be maladaptive to do this because, apart from the wasted effort, it leads to imbalances in the population sex ratio (most species produce equal numbers of male and females). This means the under-produced male chicks would do especially well in the mating game and the over-produced females will have trouble finding mates.



Male (green) and female nestling at 5 weeks old, with lead author Professor Robert Heinsohn (photo: R Heinsohn)

Professor Heinsohn has been studying the Eclectus roratus parrot - native to Papua New Guinea and the Cape York area in Northern Queensland - for over ten years. The sex ratio of the birds caught his attention when he observed captive birds producing long stretches of chicks of the same sex in succession, sometimes up to thirty male chicks in a row.

Professor Heinsohn and his research team spent six months at a time in remote rainforests in Cape York studying the parrot in its natural environment.

Eclectus parrots are approximately the size of a sulphur crested cockatoo and nest in tree trunk hollows 20 - 30 metres from the ground. Unusually for birds, Eclectus chicks have distinct gender colour differences from a very young age, allowing the sole carer mother to decide the chick’s fate based on its sex within hours of hatching.

The research team found that infanticide was only happening in certain types of nest hollows. “They are not all equal,” explains Professor Heinsohn. “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.”

Professor Heinsohn said there had to be precise circumstances in which the female parrot would commit infanticide: “It was the adult females who had poor hollows who would often - if they laid a male and a female chick - get rid of the male. It was always a younger brother, and in doing that they could speed up the development of the older female chick.”

Females nest in hollows high in rainforest trees but these hollows vary greatly in quality because some are prone to flooding in heavy rain. These hollows fill with water and the eggs and chicks inside often drown.

As female Eclectus chicks fledge up to 7 days earlier than their male siblings, the adult females with poor nest hollows stand a better chance of reproductive success concentrating their maternal efforts on female chicks.

This unusual behaviour does affect the balance between the sexes in the adult Eclectus population, and should make the over-produced female chicks less valuable. “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,” Professor Heinsohn concluded.

