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. 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.
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.
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.
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."
"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."
From BBC News