Data from: 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 ratio was not related to other factors such as maternal condition, paternal condition and other two-way interactions. These results suggest that breeding birds of grasslands are sensitive to variation in habitat conditions, and provide additional evidence that grazing intensity affects avian reproduction in cryptic ways that are rarely studied. Our findings suggest that flexible management including timely rotational grazing is needed to optimize bird species reproduction and maintain ecosystem health.

家畜放牧会影响草原鸟类的生境结构以及节肢动物猎物的可获得性，并最终决定生境质量。生境质量可影响鸟类的繁殖策略（如性比调整），但目前针对放牧强度与后代性比关联的研究仍较为匮乏。本研究以青藏高原（Tibet Plateau）上的高寒草原雀形目鸟类——穗即鸟（Isabelline Wheatear, Oenanthe isabellina）为研究对象，探讨了家畜放牧强度（grazing intensity）对其次级性比（secondary sex ratio）的影响。穗即鸟的后代性比与放牧强度的二次项（(放牧强度)²）呈显著相关。在混合家畜低强度放牧区域营巢的穗即鸟，其后代雄性比例显著高于未放牧区域（0.41 vs. 0.58，Z=-2.836，P=0.005）；而其余处理组（未放牧与高强度放牧组、低强度放牧与高强度放牧组）的窝后代性比无显著差异。后代性比的变异与母体状况、父体状况及其他双因素交互作用等其他因素均无显著关联。本研究结果表明，草原繁殖鸟类对生境条件的变化较为敏感，同时为"放牧强度以鲜少被关注的隐蔽方式影响鸟类繁殖"这一观点提供了新的证据。本研究结果提示，需采用包括适时轮牧（rotational grazing）在内的灵活管理策略，以优化鸟类种群繁殖并维持生态系统健康。