Cross-continental migratory connectivity and spatiotemporal migratory patterns in the great reed warbler

Migratory connectivity describes to which degree different breeding populations have distinct (non-overlapping) non-breeding sites. Uncovering the level of migratory connectivity is crucial for effective conservation actions and for understanding of the evolution of local adaptations and migratory routes. Here we investigate migration patterns in a passerine bird, the great reed warbler Acrocephalus arundinaceus, over its wide Western Palearctic breeding range using geolocators from Spain, Sweden, Czech Republic, Bulgaria and Turkey. We found moderate migratory connectivity: a highly significant spatial structure in the connections between breeding and sub-Saharan non-breeding grounds, but at the same time a partial overlap between individual populations, particularly along the Gulf of Guinea where the majority of birds from the Spanish, Swedish and Czech populations spent their non-breeding period. The post-breeding migration routes were similar in direction and rather parallel for the five populations. Birds from Turkey showed the most distinctive migratory routes and sub-Saharan non-breeding range, with a post-breeding migration to east Africa and, together with birds from Bulgaria, a previously unknown pre-breeding migration over the Arabian Peninsula indicating counter-clockwise loop migration. The distances between breeding and sub-Saharan non-breeding sites, as well as between first and final sub-Saharan non-breeding sites, differed among populations. However, the total speed of migration did not differ significantly between populations; neither during post-breeding migration in autumn, nor pre-breeding migration in spring. There was also no significant relationship between the total speed of migration and distance between breeding and non-breeding sites (neither post-nor pre-breeding) and, surprisingly, the total speed of migration generally did not differ significantly between post-breeding and pre-breeding migration. Future challenges include understanding whether non-breeding environmental conditions may have influenced the differences in migratory patterns that we observed between populations, and to which extent non-breeding habitat fluctuations and loss may affect population sizes of migrants.