It is widely assumed that variation in fitness components has a physiological basis that might underlie selection on trade-offs, but the mechanisms driving decreased survival and future fecundity remain elusive. Here, we assessed whether physiological variables are related to workload ability or immediate fitness consequences and if they mediate future survival or reproductive success. We used data on 13 physiological variables measured in 93 female European starlings (Sturnus vulgaris) at two breeding stages (incubation, chick-rearing), for first-and second-broods over two years (152 observations). There was little co-variation among the physiological variables, either in incubating or chick-rearing birds, but some systematic physiological differences between the two stages. Chick-rearing birds had lower hematocrit and plasma creatine kinase but higher hemoglobin, triglyceride and uric acid levels. Only plasma corticosterone was repeatable between incubation and chick-rearing. We assessed relationships between incubation or chick-rearing physiology and measures of workload, current productivity, future fecundity or survival in a univariate manner, and found very few significant relationships. Thus, we next explored the utility of multivariate analysis (principal components analysis, Mahalanobis distance) to account for potentially complex physiological integration, but still found no clear associations. This implies either that a) birds maintained physiological variables within a homeostatic range that did not affect their performance, b) there are relatively few links between physiology and performance, or, more likely, c) that the complexity of these relationships exceeds our ability to measure it. Variability in ecological context may complicate the relationship between physiology and behavior. We thus urge caution regarding the over-interpretation of isolated significant findings, based on single traits in single years, in the literature.

中文翻译：

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欧洲椋鸟 (Sturnus vulgaris) 繁殖性能的生理预测因子

人们普遍认为，适应性成分的变化具有生理基础，可能是权衡选择的基础，但导致生存率下降和未来繁殖力下降的机制仍然难以捉摸。在这里，我们评估了生理变量是否与工作量能力或直接健康后果有关，以及它们是否介导未来的生存或繁殖成功。我们使用了 93 只雌性欧洲椋鸟 (Sturnus vulgaris) 在两个繁殖阶段（孵化、雏鸡饲养）中测量的 13 个生理变量的数据，用于两年内的第一和第二窝（152 次观察）。生理变量之间几乎没有共同变化，无论是在孵化还是雏鸡饲养中，但两个阶段之间存在一些系统的生理差异。雏鸡的血细胞比容和血浆肌酸激酶较低，但血红蛋白、甘油三酯和尿酸水平较高。只有血浆皮质酮在孵化和雏鸡饲养之间是可重复的。我们以单变量方式评估了孵化或雏鸡饲养生理学与工作量、当前生产力、未来繁殖力或存活率测量之间的关系，发现很少有重要关系。因此，我们接下来探索了多变量分析（主成分分析，马氏距离）的效用，以解释潜在的复杂生理整合，但仍然没有发现明确的关联。这意味着要么 a) 鸟类将生理变量保持在不影响其表现的稳态范围内，b) 生理和表现之间的联系相对较少，或者，更有可能的是，c）这些关系的复杂性超出了我们衡量它的能力。生态环境的变化可能会使生理和行为之间的关系复杂化。因此，我们敦促谨慎对待文献中基于单一年份的单一特征的孤立的重要发现的过度解释。