Parental correlates of offspring sex ratio in Eurasian Oystercatchers

We investigated hatchling and fledgling sex ratios in Eurasian Oystercatche rs (Haematopus ostralegus) using random amplified polymorphic DNA markers. The overall hatchling (53% males, n = 374 hatchlings from 177 broods) and f ledgling (49% males, n = 51) sex ratio did not differ significantly from un ity. Hatchling and fledgling sex ratios were not correlated with laying dat e, clutch size, brood size, egg-laying sequence, territory quality male age , or male breeding experience, but hatchling sex ratio was positively corre lated with age and breeding experience of females (0.05 < P < 0.075, n = 71 ). Older females produced more sons irrespective of the position of the off spring in the egg-laying sequence. Fledging mass was not correlated with fe male age, so the Trivers and Willard (1973) hypothesis is unlikely to expla in our results. Sons dispersed less than daughters, so the local resource c ompetition hypothesis of Clark (1978) might apply. The adaptive significanc e of a male-biased sex ratio in clutches produced by older females is specu lative because the costs and benefits of dispersing versus philopatric offs pring to parents and offspring are largely unknown.