Data for: The spatial consistency and repeatability of migratory flight routes and stationary sites of individual European nightjars based on multiannual GPS tracks

Background: The degree to which avian migrants revisit the same sites to replicate routes from previous years has received more and more attention as the possibilities of tracking small to medium avian migrants over multiple annual cycles have improved. Repeated measurements of individuals can potentially inform about their navigation and migration strategies and to what extent observed variation within and between individuals may reflect the selective potential in the population.

Methods: We analysed the annual space-use of European nightjars (henceforth nightjars) Caprimulgus europaeus tracked with GPS-loggers in multiple years between northern Europe and southern Africa. We quantified site-fidelity to stationary sites and consistency, repeatability, and latitudinal correlations in route choice as well as investigated barrier associated changes of within- and between individual longitudinal variation in flight routes.

Results: We found that the nightjars consistently used the same breeding and wintering sites but that individual migratory route choice was flexible, although largely repeatable. Thus, rather than follow any flyway common to the study population nightjars followed individual-specific flight corridors while allowing for variation of a few hundred kilometres in route in both autumn and spring. Although the overall distribution of tracks was highly autocorrelated in both seasons we detected variations in longitudinal spread associated with the passage of ecological barriers enroute. Potential intermediate goal areas were detected prior to the crossing of the Mediterranean Sea and the Sahara Desert in both seasons. In spring, within-individual route variability dropped to a few tens of kilometres at the initiation of the desert crossing but increased to maximum over the barrier.

Conclusions: The nightjars incorporate an individual-specific space-use within their annual cycle that allow for a degree of flexibility during migration, possibly driven by the energetical benefits of allowing adaptive wind drift while airborne. Our data demonstrate how topography and spatial carry-over effects influence flight path variability that may dimmish or reinforce individuality in route choice. Hence, this study highlights that identifying and quantifying past and present external influences on behaviour and realised routes can be critical to distinguishing the genetic basis and variation in migration.

Methods
GPS sampling by dataloggers on individual birds during the annual cycle to track migration and non-breeding movements.

## 背景
随着多年度周期内中小型候鸟追踪技术的不断完善，候鸟是否会重返往年栖息位点以重复过往迁徙路线这一问题，正受到学界越来越多的关注。对个体进行多次监测，可为揭示其导航与迁徙策略提供依据，同时可阐明个体内部及个体间的观测差异在多大程度上反映了种群中的选择潜力。

## 研究方法
本研究针对欧洲夜鹰（下文简称夜鹰，*Caprimulgus europaeus*）展开分析，该物种的个体通过搭载GPS定位器（GPS-loggers）被追踪了多个年度的迁徙活动，其迁徙路线覆盖北欧至南非的范围。我们量化了夜鹰对固定栖息位点的位点保真度（site-fidelity），以及迁徙路线选择的一致性、可重复性与纬度相关性；同时探究了生态屏障（ecological barriers）相关的个体内及个体间飞行路线经度变化的变化特征。
补充实验方法：通过在个体鸟类的年度周期内搭载数据记录仪进行GPS采样，以追踪其迁徙及非繁殖期的移动轨迹。

## 研究结果
本研究发现，夜鹰会持续使用相同的繁殖与越冬位点，但个体的迁徙路线选择虽整体具有可重复性，却仍具备一定灵活性。与研究种群共享的迁徙通道（flyway）不同，夜鹰会遵循个体专属的飞行廊道（flight corridors），且在春秋两季的路线均可存在数百公里的波动。尽管两季的轨迹整体分布均呈现高度自相关，但我们仍检测到了途经生态屏障时的经度扩散变化。在两季穿越地中海与撒哈拉沙漠之前，均观测到了潜在的中途目标区域。春季迁徙时，个体内路线变异在开始穿越沙漠时降至数十公里，但在穿越沙漠屏障的过程中升至最大值。

## 研究结论
夜鹰在年度周期内采用个体专属的空间利用模式，这使其在迁徙过程中具备一定的灵活性，这种灵活性可能源于飞行时允许适应性风飘移（wind drift）所带来的能量收益。本研究数据揭示了地形与空间携带效应（spatial carry-over effects）如何影响飞行路径变异——这种变异或削弱或强化路线选择的个体特异性。因此，本研究强调，识别并量化过往与当前外部因素对行为及实际迁徙路线的影响，对于解析迁徙行为的遗传基础及其变异至关重要。