Joint evolution of the biogeography and phenology of seasonal migration

In migratory species, the temporal phases of the annual cycle are linked to seasonally shifting geographic ranges. Here, we investigate the spatiotemporal structure of the annual cycle in a phylogenetic comparative framework by developing a method to demarcate the pacing of annual cycle stages using eBird, a massive avian occurrence dataset, and applying it to migratory passerine birds breeding in North America. The analyses focus on 150 species of migratory passerine birds breeding in North America, filtered from a starting list of 206 species based on data quality. The analyses use eBird data averaged across the years 2002-2021 to calculate daily distributional centroids of the geographic range of each species. Next, the centroids are used to estimate demarcations of the annual cycle stages for downstream analyses. Our analyses reveal a striking negative correlation between the durations of the breeding versus nonbreeding stationary periods, indicating that a tradeoff between the lengths of the two stationary periods is the primary axis of variation in annual cycle pacing. Our results further show that the duration of annual occupancy in the breeding versus stationary nonbreeding ranges predicts the geographic separation of these seasonal ranges, demonstrating that the ratio of time spent on stationary breeding versus nonbreeding locations evolves in tandem with a species’ migration distance. By contrast, the amount of time during which species undergo seasonal migration—that is, the duration of the seasonal periods when species’ geographic ranges shift latitudinally—varies relatively little across species compared to the length of the stationary periods. Our study helps untangle the complexity of seasonal distributions and schedules to reveal integrated evolution of the biogeography of the migratory cycle, its pacing, and life history tradeoffs among species.

在迁徙物种中，年度生活周期的时间阶段与随季节变动的地理分布范围紧密关联。本研究依托eBird（大规模鸟类观测数据集）开发了一套划定年度生活周期各阶段节奏的方法，并将其应用于北美繁殖的迁徙鸣禽，旨在从系统发育比较框架下探究年度生活周期的时空结构。
本研究的分析对象为初始候选列表中的206种鸟类，经数据质量筛选后保留150种北美繁殖迁徙鸣禽。分析采用2002至2021年的eBird日均数据，计算每个物种地理分布范围的每日分布质心。随后，借助这些分布质心估算年度生活周期各阶段的分界点，以供后续分析使用。
分析结果显示，繁殖静止停留期与非繁殖静止停留期的时长之间存在显著负相关关系，表明两类静止停留期的时长权衡是年度生活周期节奏变异的核心轴。研究结果还表明，物种在繁殖栖息地与非繁殖静止栖息地的年度停留时长之比，可以预测这两类季节性栖息地的地理间隔，即物种在繁殖与非繁殖静止停留地的耗时比会随其迁徙距离协同演化。
与之相对，相较于静止停留期的时长，物种经历季节性迁徙的时长，即其地理范围沿纬度移动的季节性阶段时长，在不同物种间的差异相对较小。
本研究有助于厘清季节性分布与物候节律的复杂性，揭示迁徙周期的生物地理学特征、其节奏调控规律以及物种种间生活史权衡之间的协同演化机制。