Nestling sex ratios in the polygynously breeding Corn Bunting Miliaria calandra

We used a PCR-SSCP assay to sex 288 nestling Corn Buntings Milaria calandra from 113 clutches studied between 1988 and 1990 in the Western Isles, Scotland. In each year the sex ratio was not statistically different from 1:1. Brood sex ratios did not deviate significantly from the binomial distribution when tested by robust randomisation tests. Although the effect was not strong, the proportion of males in broods declined as the breeding season progressed in 1989, although the opposite trend was observed in 1988 and 1990. Overall, we found no effect of year, timing of breeding, levels of polygyny, brood size or female size on brood sex ratios although several terms had significant interactions with year due to the effects of the data collected in 1989. Independently of brood size, the feeding rates of males, females or both combined did not vary in relation to brood sex ratios. However, the proportion of feeds provided by fathers was highest at male-biased broods when brood size was small (1-3 young) but at female-biased broods when brood size was large (4-5 young).