Population specific annual cycles and migration strategies in a leap-frog migrant

A common migratory pattern in birds is that northerly breeding populations migrate to more southerly non-breeding sites compared to southerly: a pattern called leap-frog migration. Not only do populations experience differences in migration distances, but also different environmental conditions, which may vary spatiotemporally within their annual cycles, crating distinctive selective pressures and migratory behaviors. Information on how populations schedule their annual cycles according to environmental conditions and by extension, adapt their migratory behaviors is important to understand drivers of migration and evolution of migration patterns at large. Here we use light level geolocators and citizen science data on regional spring arrivals to compare two populations of common ringed plover Charadrius hiaticula – a textbook example of a leap-frog migrant – breeding at different latitudes. We 1) describe and characterize the spatiotemporal patterns of the annual cycles, and 2) test predictions regarding speed and timing of migration derived from the population specific characterizations of the annual cycles. Winter distributions followed a leap-frog migration pattern, i.e. northern breeding population (NBP) wintered in W Africa and the southern (SBP) mainly in W Europe. The annual cycles were shifted temporally so that the NBP was always later in all stages compared to the SBP. The SBP spent more than twice as long time in the breeding area, but there was no difference in winter residency. There was no difference in migration duration, nor migration speed, between the two seasons within population, but the NBP spent more time on migration in both seasons and migrated faster in spring compared to the SBP. We also found a larger variation in spring arrival times across years in the SBP. This suggests that a complex interaction of population specific timing and variation of breeding onset, length of breeding season and proximity to the breeding area shape the annual cycle and migratory behaviors.

鸟类常见的迁徙模式为：与南方繁殖种群相比，北方繁殖种群会迁往纬度更靠南的非繁殖栖息地，该模式被称为跳越迁徙（leap-frog migration）。不同种群不仅迁徙距离存在差异，还会经历不同的环境条件——这些条件在其年度周期内随时空动态变化，进而形成独特的选择压力与迁徙行为。阐明种群如何依据环境条件规划年度周期，并由此调整迁徙行为，对于理解迁徙驱动因素及迁徙模式的整体演化具有重要意义。

本研究利用光感地理定位器（light level geolocators）与区域春季到达时间的公民科学数据，对两个不同纬度繁殖的环颈鸻（Charadrius hiaticula）种群开展对比分析——环颈鸻是跳越迁徙的经典教科案例。本研究旨在：1）描述并刻画年度周期的时空格局特征；2）基于种群特异性的年度周期特征，验证关于迁徙速度与迁徙时间的预测假说。

研究结果显示，越冬分布呈现跳越迁徙模式：北方繁殖种群（northern breeding population, NBP）在西非越冬，而南方繁殖种群（southern breeding population, SBP）主要在西欧越冬。年度周期在时间上存在显著偏移，即北方繁殖种群的所有生命阶段均晚于南方繁殖种群。南方繁殖种群在繁殖地停留的时长是北方繁殖种群的两倍以上，但两者的越冬居留时长并无显著差异。同一种群内的两个迁徙季，迁徙时长与迁徙速度均无显著差异，但北方繁殖种群在两个迁徙季的迁徙耗时均更长，且春季迁徙速度显著高于南方繁殖种群。此外，我们发现南方繁殖种群的年度春季到达时间在不同年份间的变异程度更大。

这表明，种群特异性的繁殖起始时间、繁殖季节长度以及与繁殖地的空间距离之间的复杂相互作用，共同塑造了种群的年度周期与迁徙行为。