Data from: Connecting the dots: Stopover strategies of an intercontinental migratory songbird in the context of the annual cycle

The phases of the annual cycle for migratory species are inextricably linked. Yet, less than five percent of ecological studies examine seasonal interactions. In this study, we utilized stable hydrogen isotopes to geographically link individual black-and-white warblers (Mniotilta varia) captured during spring migration with breeding destinations to understand a migrant's stopover strategy in the context of other phases of the annual cycle. We found that stopover strategy is not only a function of a bird's current energetic state, but also the distance remaining to breeding destination and a bird's time-schedule, which has previously been linked to habitat conditions experienced in the preceding phase of the annual cycle. Birds in close proximity to their breeding destination accumulate additional energy reserves prior to arrival on the breeding grounds, as reflected by higher migratory condition upon arrival, higher refueling rates measured via blood plasma metabolites, and longer stopover durations compared to birds migrating to breeding destinations farther from the stopover site. However, late birds near their breeding destination were more likely to depart on the day of arrival (i.e., transients), and among birds that stopped over at the site, the average duration of stopover was almost half the time of early conspecifics, suggesting late birds are trying to catch-up with the overall time-schedule of migration for optimal arrival time on the breeding grounds. In contrast, birds with long distances remaining to breeding destinations were more likely to depart on the day of arrival and primarily used stopover to rest before quickly resuming migration, adopting similar strategies regardless of a bird's time-schedule. Our study demonstrates that migrants adjust their en route strategies in relation to their time-schedule and distance remaining to their breeding destination, highlighting that strategies of migration should be examined in the context of other phases of the annual cycle.

迁徙物种的年周期各阶段紧密关联。然而，仅有不足5%的生态学研究关注季节间的相互作用。本研究利用稳定氢同位素（stable hydrogen isotopes），将春季迁徙期间捕获的黑白林莺（black-and-white warbler，Mniotilta varia）个体与其繁殖目的地进行地理匹配，以期在年周期其他阶段的背景下解析迁徙者的中途停留策略。研究表明，中途停留策略不仅取决于鸟类当前的能量状态，还与剩余繁殖距离以及其迁徙时间节律相关——此前已有研究证实，该时间节律与年周期前一阶段所经历的栖息地条件存在关联。相较于飞往距离停留点更远繁殖地的个体，临近繁殖目的地的鸟类在抵达繁殖地前会积累更多能量储备，具体表现为抵达时的迁徙状态更佳、通过血浆代谢物（blood plasma metabolites）测得的补能速率更高，以及更长的中途停留时长。不过，临近繁殖地的晚到个体更倾向于在抵达当日即离境（即过境个体transients）；且在该站点停留的个体中，晚到个体的平均停留时长仅为早到同种个体的近一半，这表明晚到个体正试图追赶整体迁徙时间节律，以确保在繁殖地的最优抵达时间。与之形成对照的是，剩余繁殖距离较远的个体更倾向于在抵达当日离境，且主要通过停留进行休整后便快速重启迁徙，无论其自身的时间节律如何，均采用相似的策略。本研究证实，迁徙者会根据自身的时间节律以及剩余繁殖距离调整途中策略，凸显出迁徙策略的研究应当置于年周期其他阶段的背景下开展。