By Victoria Gill
Science reporter, BBC News
Females judge prospective mates by the colour of their feathers
Female Gouldian finches "decide" to have more male chicks if they are less compatible with their mate.
The birds, which have either red or black heads, prefer to mate with males with the same head colouring, as this signifies a better genetic match.
Chicks from a mismatched mating - particularly the females - are weaker and more likely to die very early.
A report in the journal Science says that the birds compensate for this by having more male chicks in their brood.
Colourful Gouldian finches can judge if a mate is genetically compatible just by looking at its head.
Broods are more gender balanced when the parents are the same colour
A female that mates with a male with the same colouring lays eggs that hatch much healthier chicks.
This new study has found that, when the female finches mate with a male that has a different head colour, they select the sex of their offspring - giving their chicks a better chance of survival.
In birds, the sex of an egg is already determined before it is fertilised by the male.
Sarah Pryke, a biologist from Macquarie University in Sydney, led this study. She found that when female finches mate with mismatched males, 70% of their chicks are male.
This is beneficial for the birds, because male chicks from genetically mismatched parents are more likely to survive than females.
"It is pretty amazing to think that the female herself has so much control - subconsciously of course - over this basic physiology," said Dr Pryke.
The results were particularly striking because colour-matched matings, which result in much healthier broods, always produce roughly equal numbers of male and female chicks.
"Females really don't want to mate with a male with a different head colour.
"But there simply aren't enough compatible males, so later in the mating season they seem to use this control to make the best of a bad situation."
Birds of a feather
Dr Pryke's team disguised some of the male finches to show that this "sex bias" is entirely controlled by the females.
They blackened the head feathers of red males, using a non-toxic dye, and paired them to both red and black females to allow them to breed.
"It's actually quite hard to tell the experimentally blackened birds apart from natural black males," explained Dr Pryke.
The birds were fooled, and the team found that black females that mated with the "disguised" red males produced an equal ratio of male and female chicks.
"This is the clearest and perhaps most extreme example of sex biasing that has been found," said Dr Pryke. "It's really black and white - or in this case black and red."
She said that exactly how the birds select the sex of their eggs is still a "big mystery".
"We have an idea that hormones may play a role - but that's a working hypothesis we're looking to test."
Dr Ruedi Nager, a biologist from Glasgow University who specialises in avian reproduction described this as an "excellent experiment".
"It's now clear that the control is driven by the females," he told BBC News.
"Somehow the female recognises the sex of the follicle [or egg cell] and selects it based on how much she likes the male.
"Hopefully, this will reinvigorate the debate about how this works."