DOES SEX-SPECIFIC DUCKLING MORTALITY CONTRIBUTE TO MALE BIAS IN ADULT COMMON EIDERS?

Abstract

Abstract Adult sex ratios of waterfowl often show male bias, usually explained by differential female mortality during breeding. However, the sex ratio among first-winter Common Eiders (Somateria mollissima) in Europe is already male biased; thus, the breeding mortality hypothesis cannot be the sole explanation for this phenomenon. To clarify when the observed male bias originally arises, we studied the sex ratio of hatched eider ducklings and the sex ratio of ducklings found dead prior to fledging in a wild and free-ranging population. The hatching sex ratio did not deviate from equal (50% females, n = 418). In contrast, the sex ratio of duckling carcasses was female biased (59%, n = 118), suggesting that survival of female ducklings is lower than that of males. Consequently, the adult-male bias found in many ducks may be established already during the early phases of life, despite an equal sex ratio at hatching.