For species in which sex is genetically determined, such as birds and mammals, it was originally thought that the sex ratio at fertilisation has to be 50:50, and that any adjustment in production of sons and daughters could only come after fertilisation has taken place, such as by differential survival of male and female embryos in the nest or womb.

Nager, R. G., Monaghan, P., Griffiths, R., Houston, D. C.

"We predicted that when females are in poor condition, and consequently producing low quality eggs, the effect on offspring survival would be greater in male than in female offspring and females would adjust offspring sex ratios accordingly."

Working with lesser black-backed gulls in the wild, David Houston and Pat Monaghan, experimentally mimicked good and poor environmental conditions, and examined the effect on offspring survival and on offspring sex ratio in newly laid eggs. They found that the survival of male, but not female, young was substantially reduced if they came from eggs produced by mothers in relatively poor condition.

It was demonstrated that when environmental conditions are poor, giving rise to poor maternal condition, mothers would produce more daughters.

This has since been found in several bird species but it is still not clear exactly how birds manage to control offspring sex even before development has started.