Remnant Rhine delta population of Great Reed Warblers maintains high diversity in migration timing, stopping sites, and winter destinations

Many Afro-Palearctic songbird migrants have declined, with conservation efforts mainly focused on the restoration of breeding habitat. However, pressures outside the breeding season might play a role. This includes the possibility that local relict populations no longer maintain the original phenotypic variation in migration patterns, with a loss of flexibility. The Great Reed Warbler Acrocephalus arundinaceus kept stable population levels in central and eastern Europe, but was almost extinct in the western part of the breeding range. In the Rhine delta the population declined from 10,000 individuals around 1950, to fewer than 100 at present. Here we document migratory timing, routes, and destinations of members of this remnant songbird population. It turned out that the remaining adults still showed high diversity in migratory phenotypes. This is even true in each of the two last tiny subpopulations in the Netherlands (of 50 and 15 pairs). So, even very small populations maintain the possible adaptive phenotypic variation, and with hindsight this justifies the breeding habitat restoration efforts currently underway.