Fledgling sex ratios in relation to brood size in size-dimorphic altricial birds

In six species of dimorphic raptors (females larger than males) and one passerine (males larger than females), the sex ratio at fledging varied systematically with brood size at fledging. In all species the strongest bias toward the smaller sex was established in the largest as well as the smallest broods; a more even distribution of males and females was observed in broods of inter-mediate size. We explored a specific differential mortality explanation for this sex ratio variation. Our hypothesis postulates that variation in mortality is caused by differences 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 predict the frequency of surviving males and females at fledging, assuming an even sex ratio at hatching and random mortality with respect to both sexes within broods. The model quantitatively fits the marsh harrier data well, especially in broods origi-nating from large dutches. Although we anticipate that other mechanisms are also involved, the results support the hypothesis of sex-ratio-dependent mortality, differential between broods, as the process generating the observed brood-size dependence of fledgling sex ratios in sexually dimorphic birds. Key words: brood size, marsh harrier, sex ratio, sex specific mortality, sexual dimorphism. [Behav Ecol 9:287-296 (1998)] Mechanisms causing biased sex ratios in avian broods mayeither concern the primary sex ratio—for instance, by nonrandom meiosis (Howe, 1977)—or sex-dependent mor-tality during development (Krackow, 1995). Evidence for skewed overall population sex ratios at hatching is limited (Chitton-Brock, 1986; Slagsvold et aL, 1986), although devia-tions from parity have been observed, varying with laying date, maternal age, and territory quality (Blank and Nolan, 1983