FLEDGLING SEX-RATIOS IN RELATION TO BROOD SIZE IN SIZE-DIMORPHIC ALTRICIAL BIRDS

In six species of dimorphic raptors (females larger than males) and on e passerine (males larger than females), the sex ratio at fledging var ied systematically with brood size at fledging. In all species the str ongest bias toward the smaller sex was established in the largest as w ell as the smallest broods; a more even distribution of males and fema les was observed in broods of intermediate size. We explored a specifi c differential mortality explanation for this sex ratio variation. Our hypothesis postulates that variation in mortality is caused by differ ences in food demand between broods of the same size, due to their sex composition. Data from the marsh harrier Circus aeruginosus on gender -related food demand and overall nestling mortality were used to predi ct the frequency of surviving males and females at fledging, assuming an even sex ratio at hatching and random mortality with respect to bot h sexes within broods. The model quantitatively fits the marsh harrier data well, especially in broods originating from large clutches. Alth ough we anticipate that other mechanisms are also involved, the result s support the hypothesis of sex-ratio-dependent mortality, differentia l between broods, as the process generating the observed broad-size de pendence of fledgling sex ratios in sexually dimorphic birds.