Offspring sex ratio in Eurasian Kestrel (Falco tinnunculus) with reversed sexual size dimorphism

Fisher's theory predicts equal sex ratios at the end of parental care if the cost associated with raising offspring of each sex is equal. However, sex ratios have important evolutionary consequences and are often biased for many factors. Reported sex ratios are often biased in raptors, which display various degrees of reversed sexual dimorphism, but there seems no consistent pattern in their offspring sex ratios. In this study, we investigated the offspring sex ratio of the Eurasian Kestrel (Falco tinnunculus) and tested whether the patterns of biased sex ratios were related to laying order, egg mass, hatching order, laying date or clutch size. The brood sex ratio of the Eurasian Kestrel (male) in eggs was 47.0%, not statistically biased from 0.5, but in fledglings it was 40.8%, significantly biased from 0.5 (p=0.029). At population level, both primary and secondary sex ratios did not depart from parity. We found that clutch size and egg mass affected the secondary brood sex ratio, i.e., the larger the clutch size, the larger the number of males and eggs producing sons were heavier than eggs producing daughters. Laying date affected both the primary and secondary sex ratios, and laying earlier is associated with a greater proportion of males.