Possible existence of large-scale brood sex-ratio manipulation in Eurasian Oystercatchers

Sex allocation theory predicts that parents may manipulate the sex ratio of their offspring in response to parental fitness or possible future fitness gains. In birds, family-level brood sex-ratio variation has been related to the external environment, the social environment, parental quality, and the timing of breeding. In this study of sex ratios in overwintering Oystercatchers, Haematopus ostralegus, in the British Isles, I show that neither the female bias in juvenile birds nor the change in sex ratio with age can be explained by differential migration. I also show that adult females have lower survival rates than males. If one sex is less numerous than the other, it is likely to have greater opportunities for breeding success. The results presented here indicate that Oystercatchers may adjust the sex ratio of their offspring in order to produce more of the rarer sex. This is the first evidence that birds may manipulate the sex ratio of their young in response to large-scale population processes