Data from: African departure rather than migration speed determines variation in spring arrival in pied flycatchers

Properly timed spring migration enhances reproduction and survival. Climate change requires organisms to respond to changes such as advanced spring phenology. Pied flycatchers Ficedula hypoleuca have become a model species to study such phenological adaptations of long-distance migratory songbirds to climate change, but data on individuals’ time schedules outside the breeding season are still lacking. Using light-level geolocators, we studied variation in migration schedules across the year in a pied flycatcher population in the Netherlands, which sheds light on the ability for individual adjustments in spring arrival timing to track environmental changes at their breeding grounds. We show that variation in arrival dates to breeding sites in 2014 was caused by variation in departure date from sub-Saharan Africa and not by environmental conditions encountered en route. Spring migration duration was short for all individuals, on average 2 weeks. Males migrated ahead of females in spring, while migration schedules in autumn were flexibly adjusted according to breeding duties. Individuals were therefore not consistently early or late throughout the year. In fast migrants like our Dutch pied flycatchers, advancement of arrival to climate change likely requires changes in spring departure dates. Adaptation for earlier arrival may be slowed down by harsh circumstances in winter, or years with high costs associated with early migration.适时的春季迁徙可提升繁殖成功率与个体存活率。气候变化促使生物响应春季物候提前等环境变化。斑姬鹟（Ficedula hypoleuca）已成为研究长距离迁徙鸣禽应对气候变化的物候适应性的模式物种，但目前仍缺乏繁殖季外个体迁徙时间安排的相关数据。 本研究借助光照地理定位器（light-level geolocators），对荷兰境内的一个斑姬鹟种群开展了全年迁徙节律变化的调查，该研究有助于阐明个体调整春季抵达繁殖地时间的能力，以追踪繁殖地的环境变化。 研究结果表明，2014年个体抵达繁殖地的时间差异，源于其从撒哈拉以南非洲出发的时间差异，而非迁徙途中遭遇的环境条件。所有个体的春季迁徙时长均较短，平均仅为2周。春季迁徙中雄性早于雌性启程，而秋季迁徙节律则会根据繁殖任务灵活调整。因此，个体在全年的迁徙节奏并非始终保持早到或晚到的固定模式。 对于这类迁徙速度较快的种群（如本次研究中的荷兰斑姬鹟），为适应气候变化而提前抵达繁殖地，大概率需要调整春季的出发时间。若要实现提前抵达的适应性调整，可能会因冬季的恶劣环境，或是早期迁徙成本高昂的年份而被延缓。