Brood sex ratio in the Kentish plover

Abstract

How and why do the mating opportunities of males and females differ in natural population of animals? Previously we showed that females have higher mating opportunities than males in the Kentish plover Charadrius alexandrinus. Both parents incubate the eggs, and males provide more brood care than females; thus it is not obvious why the females find new mates sooner than the males. In this study we investigated whether the sex-biased mating opportunities stem from biased offspring sex ratios. We determined the sex of newly hatched, precocial chicks using CHD gene markers. Among fully sexed broods, 0.461 6 0.024 (SE) of chicks (454 chicks in 158 broods) were male, and this sex ratio was not significantly different from unity. The proportion of males at hatching decreased significantly over the breeding season, which occurred consistently in all 3 years of the study. Large chicks were more likely to be males than females. Neither parental age nor body size of male and female parents was related to brood sex ratio. We also sexed a number of chicks that were caught after they left their nest (range of estimated ages 0–17 days) and found that the proportion of males increased with brood age. This relationship remained highly significant when controlling statistically for hatching date. As brood size decreased due to mortality after the chicks left their nest, these results suggest that the mortality of daughters was higher than that of the sons shortly after hatching. Taken together, our results show that the female-biased mating opportunities in the Kentish plover are not due to biased brood sex ratio at hatching but, at least in part, are due to female-biased chick mortality soon after hatching. Key words: Charadrius alexandrinus, Kentish plover, mating opportunities, parental care, seasonal trends, sex allocation, sex ratio. [Behav Ecol 15:58–62 (2004)]