FEMALE birds can influence the sex of their chicks, a team of biologists in Scotland have discovered.

Researchers at St Andrews University made the finding through adjusting the food intake of zebra finches.

They found that well-fed females of the species, which originated in Australia, were more likely to produce daughters while less well-nourished birds were more likely to have sons.

The study by the university's school of biology demonstrated that the mother-to-be could bias the sex of her offspring so that she produced more of those with the best chances of surviving and growing well.

Researchers said that in birds it was the female's egg rather than the male's sperm that determined a chick's sex.

Thus the female had the potential to determine the sex of her young by whether she ovulated male or female eggs.

''In some ways female zebra finches seem to be able to exert control over whether to produce a male or female egg depending on which of the two is most likely to be successful,'' said one member of the team.

The academics maintained that the birds were an excellent model to use in the study of sex ratio adjustment because, using molecular techniques, scientists could establish the sex of each egg soon after it had been laid.

Furthermore, all the resources given to the developing embryo were present in the egg at laying, and so the size and content of the egg were measures of the amount of resources the female had allocated to it, affecting its subsequent survival chances.

''We manipulated the diet quality of zebra finches to look at the effects of body condition on female investment,'' said a researcher.

''We found that females were able to exert a strong degree of control over the production of male and female eggs.

''When females were fed on a low-quality diet, they laid eggs that were considerably lighter than those laid when they were fed on a high-quality diet, and they also laid far more male eggs on a low-quality diet.

''This is the converse situation to that described 30 years ago for mammals, but it makes sense for zebra finches.

''Previous research has shown that under poor nutritional conditions, female zebra finches grow more slowly and survive less well compared to males.

''Therefore, females are producing more of the sex with the highest survival chances under those conditions.''

The St Andrews study adds to a growing body of evidence suggesting that female birds can adjust the number of sons and daughters they produce in relation to environmental conditions.