Better red than dead

Female Gouldian finches can adjust the sex of their offspring depending on what they perceive as genetic compatibility, Sydney researchers have found.

Gouldian finches have either red or black heads and preferentially mate with birds of the same colour, which are more genetically compatible.

When same-coloured mates are not available, female finches will mate with other-coloured birds, but will have smaller clutches and smaller eggs.

And of those eggs, over 80 per cent will be male, the Macquarie University team has found. In offspring of genetically incompatible pairs in this species, female chicks have a very high mortality rate compared to male offspring.

The team of Dr Sarah Pryke and Dr Simon Griffith of the Department of Brain, Behaviour and Evolution at Macquarie, decided to test whether females adjusted their breeding investment depending on mate choice.

They randomly paired 200 females, half with black heads and half with red, to either a black or red male. Two months and lots of chicks later, the female was then paired with a male of a different colour.

They found that females paired with a same-colour male produced roughly half and half males and females. For females paired with a different-colour male, however, there was a male offspring bias of 80 per cent.

To test whether this was through female choice rather than any intrinsic mortality effect from genetic incompatibility, they decided on a little trickery.

They painted the heads of red-headed males black, and then paired them up again.

They found that black-headed females paired to red-headed males painted black had a normal sex ratio in offspring despite the incompatible genotype.

And red-headed females paired to experimentally blackened red males still produced significantly more male chicks despite being genetically compatible.

The study, published in Science, provides empirical support for the life-history theory of females in birds.