The study, published today in Science, found a female Gouldian finch could control the sex of her chicks in response to whether her mate had a red or black head.

It found couples with different-coloured heads produced more than 80 per cent males.

Simply painting the male's head a different colour resulted in the female subconsciously changing the sex ratio of her offspring.

Macquarie University biologist and the study's lead author Sarah Pryke said the discovery would change understanding of sex determination across the animal kingdom.

''It really suggests that females have a lot more control than we ever gave them credit for,'' Dr Pryke said.

''It's kind of amazing to think about because birds have a sex chromosome determination system like we humans have.''

The Gouldian finch is one of Australia's most endangered birds, with only a few thousand remaining in their remote tropical savannah homeland.

Dr Pryke said the finches ''wore their genes on their head''. They had three possible head colours: red, black and occasionally yellow.

The researchers worked with a captive breeding colony in the Hunter Valley, mating 200 female finches with a male of the same head colour and a male of a different head colour. They established that redheads and brunettes, although of the same species, were genetically incompatible.

More than 80 per cent of daughters produced from these mixed matings died young, while males had much greater chances of survival.

''When your daughters are more likely to die than your sons, it makes sense to try and increase the number of surviving offspring you produce and you might as well overproduce sons,'' Dr Pryke said.

She said couples were also more enthusiastic to mate when their head colours matched, regardless of whether the colouring was false. Mismatched birds mated only as a last resort. ''They really don't want to pair together ... if you leave them long enough, they will ultimately breed,'' she said.

Dr Pryke said she could not explain how females actually influenced the sex of their chicks.

The phenomenon might be related to hormones and their possible effect on eggs in response to a more attractive mate.

''It's a huge big black box at the moment and a big mystery,'' Dr Pryke said.