Stopover by reed-associated warblers Acrocephalus spp. in wetlands in the southeast of the Bay of Biscay during autumn and spring passage

Understanding how migratory Acrocephalus warblers use the wetlands along the coast of the southeast of the Bay of Biscay during autumn and spring migrations is vital from both conservation and management perspectives. Our aim was to explore whether Acrocephalus warblers use the region in spring in the same way as in autumn. We used ringing data obtained from three wetlands (Adour, Txingudi and Urdaibai) during the autumn of 2011 and the spring of 2012. Overall, the migration in spring was much weaker than in autumn. The remarkable scarcity of Reed Warblers (A. scirpaceus) in spring may be due in part to the fact that they tend to pass through even later than Sedge (A. schoenobaenus) and Aquatic (A. paludicola) Warblers, although, judging from additional data, the spring passage of the Reed Warbler is still lower than in autumn. Sedge Warblers in spring apparently had shorter staying periods than in autumn, but had similar fuel loads.