Clarifying migration timing and its link with underlying drivers is fundamental to understanding the evolution of bird migration. However, previous studies have focused mainly on environmental drivers such as the latitudes of seasonal distributions and migration distance, while the effect of intrinsic biological traits remains unclear. Here, we compile a global dataset on the annual cycle of migratory birds obtained by tracking 1531 individuals and 177 populations from 186 species, and investigate how body mass, a key intrinsic biological trait, influenced timings of the annual cycle using Bayesian structural equation models. We find that body mass has a strong direct effect on departure date from non-breeding and breeding sites, and indirect effects on arrival date at breeding and non-breeding sites, mainly through its effects on migration distance and a carry-over effect. Our results suggest that environmental factors strongly affect the timing of spring migration, while body mass affects the timing of both spring and autumn migration. Our study provides a new foundation for future research on the causes of species distribution and movement.

澄清迁徙时间及其与潜在驱动因素的联系对于理解鸟类迁徙的演变至关重要。然而，先前的研究主要集中在季节分布纬度和迁徙距离等环境驱动因素上，而内在生物学特性的影响仍不清楚。在这里，我们通过跟踪 186 个物种的 1531 个人和 177 个种群获得了候鸟年度周期的全球数据集，并使用贝叶斯结构方程模型研究了体重（一种关键的内在生物特征）如何影响年度周期的时间。我们发现，体重对离开非繁殖地和繁殖地的日期有很强的直接影响，对到达繁殖地和非繁殖地的日期有间接影响，主要是通过其对迁徙距离和结转效应的影响。我们的结果表明，环境因素强烈影响春季迁徙的时间，而体重则影响春季和秋季迁徙的时间。我们的研究为未来研究物种分布和移动的原因提供了新的基础。