Body mass is a commonly used indicator of the energy stores of migratory animals and there is considerable evidence that it is a critical determinant of migration decisions and outcomes. Mean population mass often increases during the post-breeding period in most migratory species. Usually, this increase is interpreted as the result of fuel accumulation for migration based on the assumption that mean population mass mirrors mean individual mass. However, an empirical test of this assumption is lacking, and it is unknown whether the general increase in mean population mass is entirely the result of within-individual mass gain, or if it rather reflects a change in the nature of individuals in the population (mass-dependent turnover). We investigated changes in body mass during the post-fledging period of a migratory bird, the Red-necked Nightjar (Caprimulgus ruficollis), and combined longitudinal and cross-sectional data collected over 9 years to disentangle the relative contribution of individual-level (mass gain) and population-level (selective appearance and disappearance) processes. We found that the average body mass of fully-developed juveniles increased as the season progressed and that both individual mass gain and the selective disappearance of lighter individuals contributed to this increase. Contrary to the general expectations for migrants, the turnover of individuals contributed 3.5 times more to the seasonal increase in average body mass than individual mass gain. On a practical note, this differential contribution implied a discrepancy of over 40% between the time-average rates of mass gain (fuel deposition rates) estimated from population-level and individual-level data. Our study calls for caution in the use of population-level changes in body mass to make inferences about individual fuel deposition rates and, more generally, indicates that longitudinal and cross-sectional approaches need to be combined to uncover phenotype-time correlations in natural populations.

中文翻译：

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迁徙夜鹰的体重动态可以通过个体营业额和加油来解释

体重是迁徙动物能量储存的常用指标，有大量证据表明它是迁徙决定和结果的关键决定因素。在大多数迁徙物种的繁殖后时期，平均种群质量通常会增加。通常，这种增加被解释为基于平均人口质量反映个体质量的假设，迁移燃料积累的结果。然而，缺乏对这一假设的经验检验，并且不知道平均人口质量的普遍增加是否完全是个体内部质量增加的结果，或者它是否反映了人口中个体性质的变化。质量相关的营业额）。我们调查了候鸟羽化后体重的变化，红颈夜鹰（Caprimulgus ruficollis），并结合了 9 年来收集的纵向和横截面数据，以解开个体水平（质量增加）和种群水平（选择性出现和消失）过程的相对贡献。我们发现，随着季节的进行，发育完全的幼鱼的平均体重增加，而个体体重增加和较轻个体的选择性消失都促成了这种增加。与移民的普遍预期相反，个人流动对平均体重季节性增加的贡献是个人体重增加的 3.5 倍。在实际操作中，这种不同的贡献意味着从人口水平和个人水平数据估计的质量增加的时间平均率（燃料沉积率）之间的差异超过 40%。我们的研究呼吁谨慎使用群体水平的体重变化来推断个体燃料沉积率，并且更一般地说，表明需要结合纵向和横截面方法来揭示自然群体中的表型-时间相关性.