Increased reproductive effort results in male-biased offspring sex ratio: an experimental study in a species with reversed sexual size dimorphism

Adaptive sex-ratio theory predicts that parents should overproduce the more beneficial offspring sex. Based on a recent experimental study of lesser black-backed gulls, ve tested this hypothesis vith the great skua, Catharacta Ana, a bird species closely related to gulls but where females are the larger sex. When in poor body condition, the gulls overproduced daughters, the smaller and more viable sex under those circumstances. To discriminate beween a mandatory physio logical overproduction of female (i.e. non-male) eggs versus the overproduction of the smaller and presumably more viable sex, we conducted an egg-removal experiment vith the great skua. Since the males are smaller, larger size and being male are separated. Through egg removal we induced females to increase egg production effort. Eggs were sexed using a DNA-based technique. Manipulated pairs produced a significant male bias at the end of the extended laying sequence, while the sex ratio in the control group did not differ from unity. Our results present an example of facultative sex-ratio manipulation and support the hypothesis that in sexually dimorphic birds parents overproduce the smaller sex under adverse conditions.