Data from: Variable drivers of primary versus secondary nesting; density-dependence and drought effects on greater sage-grouse

Organisms seek to maximize fitness by balancing reproductive allocations against mortality risk, given selection pressures inherent to the environment. However, environmental conditions are often dynamic and unpredictable, which complicates the ability to achieve such a balance, and may require reproductive adjustments depending on prevailing conditions. We evaluated the effects of density-dependent, density-independent (drought), and individual (age, body condition) factors on nesting decisions of female greater sage-grouse in the American Great Basin. We obtained relocations and recorded reproductive histories from 287 radio-marked females over a period of 10 years, and applied these data to a multi-state model that estimated probabilities of initiating a first nest (primary nesting rate) or a second nest, given loss of a first (secondary nesting rate). This approach allowed us to evaluate the relative association between nesting rates and covariates while accounting for imperfect detection of nests. Sage-grouse primary and secondary nesting were influenced differently by density dependence and drought. Primary nesting was high and relatively constant among years despite variable drought conditions, but was negatively associated with population size (density dependence). Secondary nesting was lower and more variable compared to primary nesting, was similarly influenced by density-dependence, and was also sensitive to drought conditions. Females known to initiate second nests were in better body condition than females that only initiated first nests, and females of intermediate age had higher primary nesting rates, whereas secondary nesting was unaffected by age. Our results suggest that females were more flexible and responded more readily to changing conditions when allocating resources to second nests. These results are consistent with patterns that have been demonstrated for female allocation to clutch size in this system, and suggest that when conditions are poor second nests reflect a tipping point where reproductive costs (increased mortality) outweigh benefits (offspring reproductive value).

在环境固有选择压力的作用下，生物体通过权衡繁殖分配与死亡风险，以实现适合度（fitness）最大化。然而，环境条件往往具有动态性与不可预测性，这会提升达成上述平衡的难度，且生物体可能需要根据当前环境状况调整繁殖策略。我们针对美国大盆地（American Great Basin）内雌性大艾草松鸡（greater sage-grouse）的筑巢决策，评估了密度依赖（density-dependent）、密度非依赖（density-independent，即干旱（drought））以及个体层面（年龄、身体状况）等因素的影响。我们在10年周期内，对287只经无线电标记的雌性个体开展了位置重定位，并记录其繁殖历史；随后将这些数据应用于多状态模型（multi-state model），该模型可估算首次筑巢概率（即首次筑巢率（primary nesting rate）），以及在首次筑巢失败的前提下开展二次筑巢的概率（即二次筑巢率（secondary nesting rate））。该方法使得我们能够在纳入巢检测不完全偏差的前提下，评估筑巢率与协变量（covariates）之间的相对关联程度。大艾草松鸡的首次与二次筑巢行为，受密度依赖与干旱的影响模式存在差异。尽管干旱状况存在年际波动，但首次筑巢率始终处于较高水平且相对稳定，不过其与种群规模（即密度依赖效应）呈负相关关系。相较于首次筑巢，二次筑巢率更低且年际波动更大，同样受密度依赖效应的影响，同时对干旱状况更为敏感。已知会开展二次筑巢的雌性个体，其身体状况优于仅进行首次筑巢的个体；处于中等年龄阶段的雌性个体首次筑巢率更高，而二次筑巢率则不受年龄的影响。我们的研究结果表明，雌性个体在为二次筑巢分配资源时，灵活性更强，且能更快速地响应环境变化。上述研究结果与该系统中雌性个体窝卵数（clutch size）分配的已有研究模式一致，同时表明当环境条件不佳时，二次筑巢会达到一个临界点：此时繁殖成本（即死亡率升高）超过了收益（即后代的适合度）。