Data from: Across a migratory divide: divergent migration directions and non-breeding grounds of Eurasian reed warblers revealed by geolocators and stable isotopes

Migratory divides represent narrow zones of overlap between parapatric populations with distinct migration directions and, consequently, expected divergent non-breeding distributions. The composition of the mixed population at a migratory divide and the corresponding non-breeding ranges remain, however, unknown for many Palaearctic-African migrants. Here, we used light-level geolocation to track migration direction and non-breeding grounds of Eurasian reed warblers (Acrocephalus scirpaceus) from three breeding populations across the species’ migratory divide. Moreover, by using feathers grown at non-breeding grounds, we quantified stable isotope composition for individuals with known southwestern (SW) and southeastern (SE) migration directions. On a larger sample per population, we then assessed the proportions of SW- and SE-migrating phenotypes in each of the three populations. All tracked reed warblers from Germany and two thirds of the birds tagged from the Czech population headed initially SW. Nevertheless, about one third of the birds from the Czech site migrated towards SE. No tracking data have been obtained for the Bulgarian population. The initial migration direction determined by geolocators was a strong predictor of the non-breeding region, with SW migrants staying in West Africa and SE migrants in Central Africa. Feather δ34S and δ15N values confirmed the predominance of SW migrants in the German population, the co-occurrence of SW and SE migrants in the Czech population, and indicated a high (72%) proportion of SE migrants in the Bulgarian population. Thus, the combined approach of geolocator tracking and stable isotopic assignments provided clear evidence for the existence of a migratory divide in the southeast of Central Europe and predicted non-breeding range in Central and Central-Eastern Africa for the eastern population.

迁徙分界带（migratory divide）指具有不同迁徙方向的邻域种群之间的狭窄重叠区域，这类种群的非繁殖分布区也会因此出现分化。然而，针对众多古北界-非洲界迁徙鸟类，迁徙分界带混合种群的组成及对应的非繁殖分布范围至今仍未明确。本研究利用光感地理定位器（light-level geolocation），对横跨该物种迁徙分界带的3个繁殖种群的欧亚苇莺（Acrocephalus scirpaceus）的迁徙方向与非繁殖地进行了追踪。此外，通过采集个体在非繁殖地生长的羽毛，我们对已知西南（SW）与东南（SE）迁徙方向的个体开展了稳定同位素组成定量分析。随后，我们针对每个种群的更大样本量，评估了3个种群中SW与SE迁徙表型的占比。所有来自德国种群的追踪个体，以及捷克种群中三分之二的环志个体，初始迁徙方向均为SW。但捷克种群中约三分之一的个体迁徙方向为SE。保加利亚种群暂无追踪数据。由地理定位器确定的初始迁徙方向可有效预测非繁殖分布区：SW迁徙个体的越冬地位于西非，SE迁徙个体则停留于中非。羽毛的δ34S与δ15N值证实：德国种群以SW迁徙个体占优，捷克种群同时存在SW与SE迁徙个体，且保加利亚种群中SE迁徙个体占比高达72%。综上，结合地理定位器追踪与稳定同位素溯源的联合分析方法，清晰证实了中欧东南部存在迁徙分界带，并预测该物种东部种群的非繁殖分布区位于中非与中非东部地区。