A refugee swims behind a raft overcrowded by migrants an refugees as a FRONTEX plane flies above near a beach on the Greek island of Lesbos November 19, 2015. Balkan countries have begun filtering the flow of migrants to Europe, granting passage to those fleeing conflict in the Middle East and Afghanistan but turning back others from Africa and Asia, the United Nations and Reuters witnesses said on Thursday. REUTERS/Yannis Behrakis