Colourful Gouldian finches can judge if a mate is genetically compatible just by looking at its head.
A female that mates with a male with the same colouring lays eggs that hatch much healthier 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."
Source: BBC News