Distribution, ecology and threat status of the Aquatic Warblers Acrocephalus paludicola wintering in West Africa

The Aquatic Warbler Acrocephalus paludicola (AW) is the only globally threatened passerine species of continental Europe. The global population decreased by > 90% during the 20th century. AWs breed in Palaearctic fen mires and sedge meadows and spend the non-breeding season in sub-Saharan Africa, but until 2007 no regular wintering site had been identified. To date, the only wintering grounds that have been discovered are in river floodplain marshes along the Senegal River in the Djoudj area, Senegal. Searches for additional wintering sites in south-western Mauritania, northern Senegal and Gambia have been unsuccessful. In Djoudj, AWs are found in extensive marshes dominated by Scirpus spp. or Sporobolus robustus, and favour shallow water and the occurrence of Eleocharis mutata and Sporobolus robustus on the microhabitat scale. Within these marshes, however, AW appear to avoid dense homogenous stands dominated by Scirpus maritimus. Estimates of the density of wintering AWs in suitable habitats range between 0.5 and 1.6 birds/ha. According to the estimated area of suitable habitat (4,000-10,000 ha with strong seasonal and inter-annual fluctuations), the Djoudj area may hold between 10 and > 50% of the global population during the non-breeding season. Ringing, molecular studies and feather isotope ratios have failed to provide conclusive evidence for connectivity between the Djoudj area and particular Palaearctic breeding populations. Based on winter records, habitat data and satellite images, we speculate that the Inner Niger Delta in Mali could be another important wintering area. A pilot project that equipped 30 AWs in the Ukraine with geolocators in 2010 may reveal more details about migration routes and lead to the discovery of currently unknown wintering sites.