Recently, there have been a number of reports documenting the ability of birds to facultatively manipulate the sex ratio of their offspring in response to a range of social and environmental factors (e.g. Our results suggest that corticosterone may be part of the sex-biasing process in birds. Varying testosterone and 17β-oestradiol had no effect on sex ratio alone, and faecal levels of these hormones did not vary in response to corticosterone. Offspring sex ratio in this species was significantly correlated with faecal concentrations of the principal avian stress hormone, corticosterone, and artificially elevated levels of corticosterone resulted in significantly female-biased sex ratios at laying. Here, we present details of an experiment in which we manipulated levels of testosterone, 17β-oestradiol and corticosterone in breeding female Japanese quail ( Coturnix coturnix japonica) using Silastic implants and looked for effects on the sex ratio of offspring produced. Hormones circulating in the breeding female are particularly sensitive to environmental perturbations, and so could provide a mechanism for her to bias the sex ratio of her offspring in favour of the sex that would derive greatest benefit from the prevailing environmental conditions. However, the mechanism by which they achieve this feat is unknown. Recent studies have shown that some species of birds have a remarkable degree of control over the sex ratio of offspring they produce.
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