Evolution of population-specific migration routes in the great reed warbler Acrocephalus arundinaceus – evidence of a novel spring migration strategy

The migration patterns of birds breeding at high latitudes have undergone major changes during the Holocene, as species expanded from small refugia following the last glaciation. Unique features of genetic migration programs and species-specific dispersal patterns have resulted in various levels of migratory connectivity. High migratory connectivity can occur when populations expanding from different refugia maintain historically distinct wintering sites, or when species expanding their breeding ranges maintain a constant migratory vector. Alternatively, species may develop novel routes to nearby wintering sites during range expansion, also leading to high migratory connectivity. Here, we analyse light-level geolocator and multisensor data logger tracks of great reed warblers Acrocephalus arundinaceus from a population at its eastern range limit in Kazakhstan. We compare their migration routes with published data from five Western Palearctic populations to understand how migration patterns have evolved as the breeding range expanded. Mitochondrial DNA data suggest that Kazakhstan was colonised from the western part of the range, but the logger data show that Kazakh great reed warblers winter in East Africa together with conspecifics from Turkey. This indicates that their migration route did not arise as a simple parallel shift of an unchanged vector-based programme, but required drastic modifications of the migratory directions to maintain African wintering quarters. A remarkable finding in our study was the detection of a novel spring migration strategy. We found that birds leave the African wintering quarters already in February to spend up to two months at an intermediate staging area in southern Iraq, half-way to their breeding grounds in Kazakhstan. We call this a two-step spring migration strategy and discuss the conditions that may promote the evolution of such a behaviour.