Nestling sex ratio in the European Starling Sturnus vulgaris

We used a PCR-based molecular DNA sexing technique to sex European Starling Sturnus vulgaris chicks from a nestbox colony at Wytham, Oxford, UK. The nestling sex ratio among 350 one-week-old chicks from 108 broods was significantly female-biased. There was no significant heterogeneity in brood sex ratio. We found no significant correlations between sex ratio and several environmental and parental variables, nor were there any significant differences in egg dimensions or growth rates of the sexes. Mortality, but not fledging sex ratio, was strongly correlated with hatching order, suggesting that the sex ratio imbalance was not due to sex-biased chick mortality. The possibility that the overall sex ratio bias is dictated by maternal adjustment of the sex ratio is discussed.