Grazing pressure affects offspring sex ratio in a socially monogamous passerine on the Tibet Plateau

Livestock grazing can affect habitat structure and availability of arthropod prey for grassland birds, and ultimately determines habitat quality. The habitat quality may affect breeding strategies (e.g. sex ratio adjustment) in bird species, but yet studies investigating grazing intensity on offspring sex ratio are still rare. In this paper, we examined the effect of grazing intensity by livestock on the secondary sex ratio of an alpine-steppe passerine, the Isabelline Wheatear Oenanthe isabellina, on the Tibet Plateau. Offspring sex ratio of Isabelline Wheatears significantly correlated with a quadratic term -- (grazing intensity)2. The wheatears nesting in areas grazed at low intensity by mixed livestocks produced significantly more sons than those in ungrazed area (0.41 vs. 0.58, Z=-2.836, P=0.005), while brood sex ratios from other treatments (ungrazing vs. intensive grazing and low-intensity grazing vs. intensive grazing) did not differ significantly. Variation in offspring sex rat...