Female Gouldian finches "decide" to have more male chicks if they are less compatible with their mate. They prefer to mate with males who have the same coloured head, as this signifies a better genetic match. Chicks that have been produced from a mismatched mating - particularly the females - are weaker and more likely to die very early.

Colourful Gouldian finches can judge if a mate is genetically compatible just by looking at its head, and 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.

The study was led by Sarah Pryke, a biologist from Macquarie University in Sydney.  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."

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.

Dr Pryke has said that exactly how the birds select the sex of their eggs is still a mystery.

"We have an idea that hormones may play a role - but that's a working hypothesis we're looking to test."